Run the Pacific Standard heist as many times as you can stand.
Or start up a high paying job and run through them nonstop on hard until you tire yourself out.
And that is why microtransactions are bad... because look, already you are advising some one to do the same thing over and over and over again until they get hte money. WHich is exactly how MTs make their money, cause people get sick of doing stuff over and over and over and eventually just want to pay to get over the grind. And they will purposely design a game to encourage that.
This is exactly what I mean when I say you can tell R* was influenced by the MTs in their game design. It encourages you to grind the same missions over and over (or heists) to try to make that money and hopes you get tired of it and decide to pay instead.