You're looking at £5 worth of product per wheel for the amateur stuff; Multiple layers of of a decent pro coating. Plus time, plus heat curing, plus disposables (applicators, cloths, etc). £25 per wheel isn't horrendous... depending on what they propose to use and how good they actually, of course.
If you're doing them yourself, C5 is difficult to beat really.
If you're paying someone else to do it, make sure you know what they're using and how good they are before you hand over your hard earned.