Another, probably safer and easier method is to simply choose a "unit cost." Let's say you chose $25. You would tell your affiliates that they can participate for $25 in your co-op ($25 for each equal share of affiliates that the co-op generates). Then simply take all the money collected and buy as much advertising as the total amount collected allows. The Co-op Manager will take it from there, equally dispersing all affiliates that your advertising generates.