There, you have some problems:
* The anonymity would be somehow compromised since the relay would be able to link different connections of yours. He still shouldn't be able to decrypt it if he's not the exit-node, nor know your IP, but anyway, sounds a bit more vulnerable than the current protocol...
* There's a trust issue too. If it's a prepaid contract, the user has to trust the relay. If it's post-payed, it's the relay that must trust the user. Maybe the best would be a prepaid service with some sort of flagging mechanism to ban fraudulent relays. But then, all the thief-relay needs to do is request a new IP to his ISP...
There is also the problem that each relay will want to be paid. This would add up rather quickly and become burdensome to those who can't afford it. Just because you can't afford it doesn't mean you shouldn't have freedom.