The most I've paid was £150 for an hour, way above what I would usually. I wanted to see if it was true that higher priced punts really are better. She did turn out to be pretty great, but I'm still too tight to pay that regularly. £60-£70 for a half hour is usually my limit, and I've had some equally great punts in that price range too.
The most I would pay is a trickier question. If I could get everything I wanted from a punt I might go up to £200, but generally speaking I'd never go over £100.