If you have a squad of homies that don't mind spending a week or so grinding the heists, the Criminal Mastermind challenge nets you $10 mil which is more than enough to get you a CEO Office, MC Club and/or a Bunker. All 3 make money for you. MC Club and Bunker make money passively while the Office stuff is things you can do actively. Selling top end cars with 3 of your homies can net you up to $400k a run, selling a full cocaine farmhouse can net you $420k, selling a full bunker can net you over 1 mil.
Making money isn't difficult in GTA, its just a matter of consistency, grinding and hoping your lobby isn't full of dikkheads that blow your shyt up during a run.