I agree with Mat, I've seen loads of names that are £10-20, usually low ticket, or items that awin tells me I've sold a dozen off over the year and made like £1-2 off each, so £10-20 seems a nice investment but then when you add nom on there, it almost doubles your ROI and makes you ummm and arrrgh.
I think a monthly cap would be a good idea, even £11.75 a time, but maximum of 5 charges per billing period much like overdrafts with your bank, would be preferable.