There are two main axes when it comes to business models for ad tech companies: Transparency and Price.
A company can be at the high end of the pricing, with zero transparency, or at the other end of the bridge, and hand over its platform to the clients, and charge them a reasonable fee for usage.
The business model depends on if it is a product company or a service company (aka agency). A product company rents its platform to the clients. An agency will have its platform but will focus on delivering results on behalf of the client.
A product company can charge you based on a number of users, or based on data consumption.
MailChimp pricing tiers are tied to how many subscribers you have. So is AdEspresso, who will charge you based on how much money you spend on Facebook, and how many CRM leads you to want to sync. On the other hand, HubSpot will charge you per user.
For service companies, they have two models: Performance and Managed Spend.
If you hire a performance marketing agency, they will charge you per results: purchases, leads, subscribers, anything that you consider valuable.
If they run the ad campaigns for you, they will take a percentage of the total spend on each channel.
Some ad tech companies will use a hybrid model. If you are looking for one, choose a company that gives you the most transparency and the lowest cost. If you are an ad tech company, clients will switch to whoever gives them more transparency in the long run… at the lowest price.