Ah, apologies, I misremembered. I can only get about 1.2 million per day per slave (in v0.3.13, though I doubt things have changed much). I'm not sure why I thought I could get double that.
In any case, to maximize profit you need a fully upgraded industrial milking room and a slave who produces all four fluids (cum, girl cum, milk, udder milk). Give all their fluids all the special properties and a non-standard flavor, make sure they produce enough of them to hit the milker cap (120.1 L/day for milk and udder milk, 24 L/ day for cum, and a drooling pussy), then chain them to the milker for 24 hours a day. They'll get stat penalties from being overworked, but since they won't be accompanying you to combat, those don't matter for squat. That's quite a lot of transformations, so I prefer to do this with a slime or a demon, since they transform freely with no need to grind the enchanting menu.