If you want to use WhatsApp as an official channel for your business to communicate with customers, you have two alternatives: Use the WhatsApp Business App if you are a business with a small customer base or WhatsApp Business API if your customer base is medium to large.

Source of the image: https://developers.facebook.com/docs/whatsapp/overview
Check this article to understand better the differences between WhatsApp for Business App and WhatsApp for Business API. Although WhatsApp for Business App is free of charge, WhatsApp for Business API is not free.
And although you might end up paying a single bill for a single provider, the price you pay is quite complex, and you need to be aware of the components of the price you pay.
This article will clarify the hidden costs of running WhatsApp for Business API, expose the available pricing models of the mainstream vendors, and guide you to choose the vendor that will fulfill your needs.
WhatsApp for Business API's total cost is composed of three elements:
WhatsApp fees for the template Messages
WhatsApp Business API hosting fees
WhatsApp Business solution providers' fees
First costing element: WhatsApp fees for the template messages:
WhatsApp provides two types of messages on its platform:
User-Initiated Messages: also called session messages in the documentation of Facebook, and happen when a customer sends a message to a business. The business has 24 hours to respond as per Facebook rules, and the 24-hour countdown starts from the time of the last message sent by the customer.
Business-initiated messages are also called notifications or templated messages throughout the documentation of Facebook. It happens when a business sends a message to an opted-in customer out of the 24 hours of the session messages.
When sending notifications, you must use a previously approved template.

source of the image: https://developers.facebook.com/docs/whatsapp/overview
WhatsApp messages pricing:
Until February 2022, User-initiated messages are free of charge.
Business-initiated messages (template Messages) are priced as per the quantity and the destination countries of the sent messages. Destination countries are considered based on the WhatsApp number country code of the customer.
All the pricing details are disclosed on the Facebook developers' website as per below.

Source of the image: https://developers.facebook.com/docs/whatsapp/pricing
Facebook doesn't allow new businesses to use the WhatsApp Business API directly and restricts access to a list of its messaging partners responsible for activating and maintaining the service. Of course, WhatsApp Business messaging partners will add up all the costs associated with the template messages billed by Facebook and add their margin, as explained in the next section.
Second costing element: WhatsApp Business API hosting fees
As mentioned earlier, Facebook only delivers the WhatsApp Business API through its messaging partners.
The primary role of Facebook messaging partners is to host the service on a dedicated server for every business and as per a specific list of requirements that WhatsApp dictates to guarantee the availability and security of the service.
In return for this service, Facebook messaging partners requests fees, and although there's no regulation on the pricing of these providers, there are three types of pricing available in the market:
Per-message pricing:
In this case, the Facebook messaging partner adds a transaction fee on top of user-initiated messages or business-initiated messages.
As per the below example, twilio is adding 0.005 USD on top of Facebook's original prices.

Source: https://www.twilio.com/whatsapp/pricing/us
Other Facebook messaging partners offer the same model but with a much lower margin, as you can see below Gupshup pricing:

source: https://www.gupshup.io/developer/whatsapp-api
You should choose this model if your number of customer conversations is relatively low (lower than 4,000 customers per month).
As per our analysis of Facebook messaging partners, Gupshup remains one of the most competitive providers regarding the pricing per session model.
BusinessChat.io is connected directly to Gupshup.
"Monthly active users" bundles pricing:
This type of pricing decreases the price per message in return for a commitment to the number of active users regardless of the number of messages they send. This type of pricing might be interesting if you can forecast the number of customers you have on WhatsApp.

Image source: https://www.cm.com/whatsapp/
As per our analysis, we don't recommend this model and prefer the unlimited usage pricing if your business has over 4,000 customers monthly over Whatsapp.
Unlimited usage pricing:
The last model is quite exciting and is considered hassle-free for businesses that don't want to worry about the number of customers or messages per conversation.

Source: https://www.360dialog.com/en/whatsapp-business/
If you are sure that your business will handle over 4000 customers per month over WhatsApp, such fixed pricing with unlimited usage will be perfect for your Business.
360dialog remains unique and the most competitive and reliable WhatsApp business API provider, per our knowledge and extensive research.
The 360dialog only constraint is that it is available if you have some development capabilities in your team or you will need the support of a third-party WhatsApp Business solution.
Suppose your third-party business solution partner is already having the WhatsApp API service available. In that case, he'll probably refuse to connect with 360dialog and try to make you pay higher fees as per the bundles we've seen earlier.
BusinessChat.io offers free-of-charge integration to 360dialog.
Note: not all the messaging partners of Facebook offer the WhatsApp Business API as a standalone service, and most of them offer it only as an integrated part of their overall solutions.
Third costing element: WhatsApp Business solution providers' fees
Like any other communication channel, WhatsApp is not self-sufficient, and you need to integrate it into your third-party solution to deliver the expected benefits.
For example, you must integrate your email channel into your ticketing system to fulfill your customer service through email. Another example is that you must integrate your voice line into your call center system to perform customer service through phone calls.
Concerning WhatsApp solutions providers, Facebook extends vendors supporting businesses to two categories BSP and ISV. To make the story short, as a customer, you must ensure that your provider has a signed contract with Facebook to protect your data and its privacy.
Tens of providers offer the service, and soon we'll have hundreds in the marketplace.
The vendors usually offer a conversational alternative to marketing, customer service, eCommerce, and clienteling software.
Of course, these solutions offer a first automation layer through bots and a second personalized layer through humans.
When it comes to pricing, we probably can't compare apples to oranges. The feature list that a customer service solution is different from an eCommerce offering.
But there is a clear pattern in the pricing of all of these vendors, which generally applies to one or two of these models.
Pricing as per the number of monthly active users:
This pricing is taken from the chatbot industry, where the cost is directly related to the number of customers that have a conversation with your business during a given month.

Source: https://messagebird.com/en/pricing/
150 USD per month for 1500 active contact is the pricing of Messagebird.
But still, this model is billing also the number of active live agents using the solution.
Pricing as per the number of customers:
This pricing is taken from the CRM and marketing automation industry, where the cost is directly related to the number of customers on the contact list.

Source: https://landbot.io/pricing
70 Euro per month for every 500 chats, If you don't need to have live agents to communicate with your customers and have capabilities to build your bot on the self-service platform, Landbot can be a good solution
Pricing as per the number of agents:
This pricing is taken from both the customer service and the ticketing industries. Therefore, the cost is directly related to the number of agents (sales or customer service) that use the solution.

Source: https://www.freshworks.com/live-chat-software/pricing/
Up to 69 USD per month per agent, Freshchat is pricing based on the number of active agents while also having a threshold of the number of free communication that you can have on the chatbot.
Worry-free communication pricing:
Although we've not seen many vendors adopting this model, businessChat.io offers unlimited chat on all its chatbot packages.

Source: https://www.businesschat.io/pricing
For less than 50 USD, you can get unlimited chatbot conversations with an unlimited number of customers and three free-agent consoles for use by the customer service or sales team.
The packages are scaled as per the sophistication needed in the chatbot. In addition, three live agents are also offered free of charge.
Conclusion
Once you've got the offers from different vendors, you need to compare apple to apple before closing a contract. Still, you always need to ensure that you're confident about your requirements:
The number of template messages monthly.
The number of customers that might contact you monthly.
The average number of messages per conversation.
The level of sophistication you need in your chatbot
The number of live agents that need to get in touch with your customer.