Unfortunately, there is no free market in medical training - supply is artificially capped. A huge number of qualified people who want to become a doctor are turned away from medical school, and the government artificially restricts the number of medical schools (at the behest of the AMA) [1].
[1] Some numbers. I'll assume the average black medical student is qualified (but the 49'th percentile is unqualified), giving me cutoff of MCAT VR 8.3 for a "qualified doctor". This cutoff is chosen so I can call people who disagree with me a racist. Some gaussian-fu suggests we could train at least 10-15k more doctors per year.
http://web.archive.org/web/20080801022539/http://www.aamc.or...
(Unfortunately the AAMC took the numbers down, but the wayback machine is helpful.)
That's the rationalization a lot of professionals are making.
Similarly, if you're offering a rare/unique service that is in high demand, and that only you personally can provide, and you start to become overworked, do you raise rates? If you want time off, would it not make sense to raise rates until you have a better work/non-work time balance?