> They solve the transaction cost problem by making you buy $20 worth of points at a time...

The transaction fees on a one time purchase of $20 is cheaper than on 15+5 or $1 20 times or even $0.05 400 times. The problem then is if the users would pay that much in advance, or if you can wait until that much as accumulated until you charge them for it. Then again there are services like tipjoy that manage that for you.