If you want to make money...
1. Don't buy property or anything else outside of Weapons & Ammo.
I make sure each character has...
Sniper Rifle with advanced Scope
Pistol with Suppressor
IMO those are the only purchases neccesary. The rest of your weapons you get off of dead enemies
2. Finish "The Big One" I got $34,272,000 for each character
3. Do the Lester Assaination Missions and invest heavily in your work.
There are a total of 5 Lester Missions but you can only save 4 for after the final heist.
For missions 2,3 & 5 invest all character's money before the mission.
For mission #4 invest all 3 character's money after the mission.
Lester Mission #2 Invested $36,283,216 in Debonaire cashed out with $65,871,700
Lester Mission #3 Invested $65,871,604 in Fruit cashed out with $82,339,752
Lester Mission #4 Invested $82,339,736 in Vapid cashed out with $164,682,768
Lester Mission #5 Invested $164,682,752 in Gold Coast cashed out with $298,024,864
If you do it this way all 3 character's will have a minimum of 297 million each.
Then you are free to go complete the side missions and buy eveything your heart desires.