Okay, so this is a VERY easy step-by-step way to earn ALOT of money in GTA.

(NOTE: You will only get $10,000,000+ if you have completed the story mode and have loads of money. Also you may earn a bit less than $10,000,000 but not much)

(NOTE 2: I DID NOT FIND THIS METHOD, AND YES IT IS LEGIT, YOU WILL NOT GET BANNED!)

Step 1. Bring up your phone, and go to LCN Stocks and put all of your money into AuguryInsurance (AUG).

Step 2. Go to the streets and go on a giant rampage blowing up every car you see, and do so for about 3-4 minutes.
(I would suggest using cheats such as:
Explosive bullets
Slow Motion aiming (for better accuracy)
Lower Wanted Level or Invincibilty (so the cops dont slow you down)

Step 3. Then go back to your house/apartment after doing the first 2 steps, keep looking at how much profit you have earned, after every time you look, go to sleep in your bed (if offline).

Step 4. Once the profit gets close to £10,000,000, sell all your stocks in AuguryInsurance, then there you go, just keep repeating all these steps to become one rich mofo.