As the currency balance is in here, right now the most profitable way to play the game seems to be Lumberjack simulator. As cutting wood with an axe and selling it is all you really need to do, it's way more profitable than the regular post-apocalyptic things you'd want to motivate the player to do, and much safer.
Taking a risk and exploring to find slaves to sell? They sell for like 25, you get more than that by cutting wood once. (I can't remember, but you should also be able to sell male slaves too) Become a pit fighter and make money that way? Maybe at higher ranks it's worth it, but as of right now cutting wood is way better.
Basically wood should trade 2:1, slaves/fights/prostitution should reward more credits, as I see it.