they are happy with a tip of 100 bath. [sic]
100 baht means that they won't actually starve to death that day. And the 60 baht commission from the shop's fee will go towards their rent.
I'm a big fan of chair massage and Thai massage, and over the years I've gotten to know a lot about the massage business. Unlike tourists, I understand that the money paid to the masseur is not a "tip". Because the commission is so low, what's paid directly to the masseur is the bulk of his income. Not once have I ever paid less than 200 for the "tip".