I'm guessing the problem is that since you dont know how long users will stay on the monthly contract for, you dont know how much each user , on avg, will make you ergo don't know what to pay affiliates for commission?
Easy solution. Have your commission be a recurring commissions. Basically they get X% of each recurring payment. That way the marketers are trying to target users who will stay on board for a long time (ie. make them more commission).