Human trafficking, which involves slavery of both the sexual and non-sexual type, is perhaps the most profitable criminal venture to engage in, explosive in growth opportunity, all the more so because a lot of countries look the other way on it more than on drugs or arms-smuggling or what have you.
The cost to keep people alive and healthy enough to give you a return is lower than you might think, especially since in the case of prostitutes you can just have your girls "arrested," then the state foots the bill for fixing whatever medical problems they might have, if minor enough to fix, then you get around to bailing them out
There is no question of viability
EDIT: Ninja'd by Netyaroze who gave the answer more comprehensively, but yeah.
Monster Hunter: pissing me off since 2010.
