I don't think it's very common for a reader at a shop to receive the entire reading fee. The shop where I read gets a percentage of everything I earn while reading there, which around here anyway is the norm, although the percentage differs. Also, I imagine that in some cases, the reader sets her own fee, and in others, the shop may set the fee, which makes a difference as well.
I do receive tips quite often, which are much appreciated, although I never suggest or hint that my clients should do so, and I do not treat the ones who tip differently from those who do not.
The amount of the tips I receive varies greatly; there doesn't seem to be any rule about percentage that people follow.
If you are in doubt, call ahead and ask the shop, or ask the reader when you go there. People ask me quite frequently if I accept tips.