The solution here is to use a hybrid model. Users subscribe for $20/month (and that is their cap). And each page they visit they get dinged $0.01 (etc) and the

The solution here is to use a hybrid model. Users subscribe for $20/month (and that is their cap). And each page they visit they get dinged $0.01 (etc) and they have no risk of going over their cap. When they hit a page with ads, the publisher site hides ads if they get paid $0.01, but they display ads if the users account is down to zero for that month.