Those obscene profits seem to just be a result of the numbers/math used to calculate them being placeholders.
I think the real problem is conceptual. Owning slaves in LT is one of those things that just can't help being broken in one way or another. It's passive income, it's someone else doing work and you reaping the benefits, that's the whole point of slavery as an institution. If you're just tweaking numbers, it's very difficult to find the sweet spot between "so unprofitable you have no reason to bother" and "so profitable you have no reason to do any other money-making activities", especially if the player can easily increase their profits by an order of magnitude by just getting more slaves. Sure, you could place some arbitrary restrictions on the number of slaves a player can own, but that feels, well, arbitrary. Or you could require the player to do some activity to keep the slave business going, but that would defeat the whole "get other people to do the work" point of it. You then risk running into the same issue Bethesda runs into in TES, where you still have to do everything yourself despite being the boss of an entire faction.
Ultimately I'm not sure it's really a problem that there is such a wildly unbalanced way to earn money. LT is a total power fantasy, so being able to acquite lots of money (= power) doesn't really make it any less fun. I also think it's inconspicuous enough that most people won't find it on their first playthrough, so it effectively becomes a nice shortcut to skip the boring early game on subsequent playthroughs.