Like the other PPs, bride pays if it is required. BMs pay if it is optional.
Honestly, though, I strongly lean toward it always being optional for the WP. I prefer to do my own hair and make-up. It usually looks worse when professionals try.
In fact, even for my own wedding, I did my own make-up and SHOULD HAVE done my own hair. My hair would have looked SO much better if I had done it myself and I would have saved $100. Oh well. Should have known better.