Compare Adam Smith's analysis in Wealth of Nations:

"The time and study, the genius, knowledge, and application requisite to qualify an eminent teacher of the sciences, are at least equal to what is necessary for the greatest practitioners in law and physic. But the usual reward of the eminent teacher bears no proportion to that of the lawyer or physician; because the trade of the one is crowded with indigent people who have been brought up to it at the public expence; whereas those of the other two are incumbered with very few who have not been educated at their own." - http://www.econlib.org/library/Smith/smWN4.html#I.10.94