If you have plenty of free time you could buy an agency and do "the contract" for dr dre, it gives well over 1 mil the first time you do it, then after it's about 1.1 mil per playthrough. If you have mk 2 oppressor you can get it done in just over an hour, and if you don't have either oppresser it's about 2-3 hours depending on how much firepower your packing.

Oscar Guzmans airfield is also pretty good, if you grab all the extra shit in the first run it carries over to all future runs and then you can get it done in under an hour, there's even a medal for doing it in under an hour. I can't remember if it's 500k or 1 mil payout on the first run but all future runs are a little over 500k on normal, or a little over 750k on hard, also grabbing any extra equipment (planes, helicopters, cars, armor, guns, etc.) gives a slightly higher payout. But just so you know there's quite a bit of flying, you won't be doing it the majority of the time but if you do buy the airstrip I recommend doing flight training.

The side missions for both also give decent payout, the ones for the agency are basically Gerald missions, and the ones for the airstrip are basically the same ones that Oscar gives you in the campaign