No, you can provide your co-op totally free if you wish. You could also do something like paying half of the advertising cost yourself and only charging your participants for the balance. Another idea is, rather than charging money to participate, to allow participation based simply on accumulating a certain amount of CV, or completing the Affiliate Profile, or generating their first sale, etc.
Example: You could let all your affiliates know that you will be doing $2,000 in advertising next month, and you'll split up all the affiliates generated amongst everyone who sponsors two new affiliates this month. Be creative! You can, of course, track who qualifies via your Genealogy.