I love this discussion, I've had it many times with the attendants and seen it at many spas myself.
The way it works with "most" holistic spas is the girls don't get any of the room fees. So if it's $40 for half an hour, that goes entirely to the spa. Meaning they work on tips alone. Some of the Asian places are cheaper usually because the owner does the massage herself and pockets the room fee along with whatever tip is provided so $100 total works for her. Or the owner will take a hit for the $40 room fee and pay commission to the attendant if the tip was adequate. Now I say "most" holistic spas because some pay commission and even some of the busier or stricter ones charge the attendants a shift fee.
RMT Asian locations work on commission, meaning while you'll see signs for and pay $60 for an hour RMT massage. The attendant doing the "RMT" massage won't actually have an RMT license but the receipt printed for your insurance will go through the owner's RMT license even though they didn't provide the service. I'm guessing the attendant would get a commission off that $60 plus the small tip for the massage.
Bodyrubs tend to be all-inclusive until you get in the room and want upgrades. Although some owners are strict and won't allow attendants to request more money. I'm limited to my knowledge of bodyrubs but this is my understanding of places I've been to myself.
What really makes it difficult for the attendants is explaining this to the first-time customers, especially those whose English is a little broken or lacking. It's also a little demeaning to offer a low price for a service they are providing, haggling over the typical price usually isn't a good way to start a session. This constant battle every day would put anyone in a foul mood, but most attendants have thick skin and march on dealing with the many problems that can walk in the door. I honestly think they are champs dealing with this day in and out and still staying positive and coming back to work.