What you can also do if you don't mind your girls getting gangbanged by black dudes is go to the Nigerian Prince and tell him about your problem. He will then send security for free (they gangbang your girls from time to time but no penalties afaik).Yeah the merc did the trick but the grind tho
The Nigerians aren't as good as the mercenaries but still much better than the 80 credit thugs and free.
Agreed on the grind though, I think the best method is pushing drugs at the night club.
I have 10 whores and 3 drug dens and make about 600/night from them after the 60% cut.
With selling slaves it takes multiple days to a week to break them for 2500c, Tony doesn't want to buy my product anymore (was also about 2000 per sale) and in the night club I can make 1500-2000 per night more or less. I think that's more efficient than training slaves.
As for whores, I found the easiest way was to blackmail them, followed by getting them hooked on drugs. Corruption takes forever as well.
My dealers still get busted a lot, even after I had group sex with the chief of police a few times.
This is definitely a slower game.