If your good at using the stock markets in the game you can make big money if you play your cards right.

You will get 'Random' occurrences where people may need your help like a woman's purse getting robbed or a rich guy who's going to miss his plane.

I helped this rich guy catch his plane on time and he gave me $200,000. there was another occurrence where the daughter of the mafia got ambushed and was about to be killed. If you save her she pays you a lot of money too ($50,000 if i can remember correctly). Your rich by the end of the game anyway due to certain heists that will foreplay.

Top right of Mt. Chilliad there's a road which leads to a weed farm.

Raid the weed farm (you have to kill 5 or 6 enemies) and then you go to a small green blip which gives you money ($20,000 min, $100,000 max).

You can do this a few times a week ig. I've done it once and got around $54,000.