You have two ways you can go: the users pay more, or the trainers pay more. So initially try to understand, what they are willing to pay for.

Users would be willing to pay for convenience - good trainers without having to shop around. Could you make some kind of rating system of the trainers, user driven? Or provide interesting content for fitness users, blogging, fora or something similar?

In all likelihood, users would not pay a fee to get a trainer. But if you could make them come back, and you could document that increased traffic for the trainers, they would be willing to pay more, as they would get access to more prospective clients.

For the trainers, you could give them access to blogging or facilitating in discussions to get closer user interactions. Or you could make a SaaS-like pricing model, where you limit the number of clients each trainer can get depending on their monthly fee. If you know, how many clients you provide for each trainer, you know the value you create, and you can convert that to a fixed cost every month instead of you getting money pr. transaction. But you would have to be able to document the number of prospective clients, that each trainer can expect to make new trainers sign on.

Feel free to set up a call, if you want to discuss these ideas further - or we could come up with new ones. There are many ways to go on this.

Best regards
Kenneth Wolstrup