Johnb, you are not crewing, you are not sharing expenses, you are paying tuition and if the skipper isn't running a "business" of training...the only question is whether he can get folks who are happy to pay his "tuition" fee.
If you could book similar passage with some recognized sailing school and come out of it with some type of credential, it might not be unreasonable. But to simply share a ride...some owners might at the most ask you to pay a share for food, fuel, dockage, etc. but many would be looking for crew and would take competent crew along at no charge at all.
Whatever all parties are happy with--is deemed fair. Common or otherwise.