You might be able to find some armature host that is run by an 18 year old for your price range, which is at risk at closing his company every day because he cant last in the industry with offering Reseller Plans at that rate.

If you are interested in a professional billing system they run from 15.99 - 25.99 / month and you are looking for this for free. Most bigger hosting compaines offering free billing software because when they buy like 100 Licenses they get like a 50% discount

A free billing system I can recommend is PHPCoin.

I hope I was some kind of help to you.