How much is it to rein?

If you mean the equestrian sport of reining, there are different levels. If you just want to have fun and don't want to compete seriously it would be comparable to other diciplines like huntseat or western pleasure. If you want to compete on a larger scale it will cost substancially more. The horses that are trained for this sport are expense to buy, add to that entry fees, lessons, travel, and and it could easily run $100k or more.