How much do you pay your housekeeper?

Mine charges $100 for every other week to clean my two-bedroom, up from $80 five years ago. She also washes the sheets and towels. I'm very happy with her work and I pay her a Christmas bonus every year of twice her rate, plus I give her $20 every couple of months when I have something extra for her to do. I've also referred her to lots of friends. Last week she asked me for a raise to cover what she said was an increase in her transportation costs. She didn't ask for a specific amount -- I told her I would give her $5 more dollars each time but I don't think she was too thrilled. On the other hand, I think I'm a really reasonable employer (for example, I always pay her when I go on vacation, unlike some of her clients), I'm already paying market rate (I think), and my own income has only gone down over the past year. It's not like giving her $10 or $15 raise would bankrupt me -- it's just that I think I'm paying a fair wage already. Am I? Or is the real issue that she doesn't make enough money for doing this job even if were to pay her $200, and I should just consider this a mitzvah, to help someone less fortunate?