I think yoga teachers' fees ought to reflect their training. If you spend a year or two (or more) learning from a reputable source, your wage should be commeasurate. If you took an email test online and got "certified" in an afternoon--whatever that means--that lack of diligence should be acknowledged, too. Continued training ought to be rewarded, as well, but I suspect that is more rare. And, of course, there's the issue of market value...people pay more for yoga classes, so teachers can ask more (some chicken and egg at work).
The under-paid teacher complaint is an old one. Those that want to share their knowledge and expertise often have to take satisfaction in a job well-done as part of their renumeration, whether yoga instructors or college professors or kindergarden teachers. These are crucial positions, and vital to the improvement of a civilized population, but rarely is it rewarded as it should be. The world values lawyers, businessmen, movie stars and football players more.
As far as fitness goes, I'll bet it's the aerobic instructors that get the shaft, payment-wise, more than yoga teachers, tho. Maybe the training isn't as long, but the amount of prep that it takes to choreograph and learn the routines is time-consuming. A strong background in physiology isn't required, but there has to be some familiarity with how the body works and how you train it. And then there's the level of exersion necessary to serve up a useful, engaging class to a roomful of, perhaps, less-than enthusiastic participants. Cetainly worth more than minimum wage...
So, this is what I think about. I'm not sure I have an answer...life is not fair. I wish yoga teachers (most of them) made more money than than Lindsay Lohan. And people should want to invest in the instructors that help keep them hearty and hale. But they don't, so I suppose it's pointless to spend too much energy on it.
I read a quote that's pertinent: "Santosa (or contentment, one of the niyamas from the Yoga Sutras by Pantanjali) cannot be practiced; it has to come from within. We're all discontented. The trick is to be content with that."--B.K.S. Iyengar.