Thank you for your answer :)
When a breeder posts a project on our website, we calculate with him the real costs of the foal's breeding, for the entire breeding's period until the sale.
That would include any expenses (food, veterinarian, insurance etc...)
We subtract the part that the breeder want to keep, then we divide the amount by the number of desired co-breeder.
Thus we're getting the part's price that the co-breeder will pay just once ! :)
The co-breeder will not have to pay additional fees.