Hello there, I'm here today to share with you what I do when I need money, I hope this guide can help a lot of new players.
Los Santos is the best city where to earn money... Why?
Because in Los Santos, we can find a lot of high-payout robberies... Where? "keep reading"
Los Santos has 2 Didier Sachs shops (luxury clothes shops) that can be found; 1 in Rodeo (near the bank) and 1 in Los Santos Airport, in each shop you can get 3.000$ to 6.000$ by robbing the cashier, and 7.000$ to 13.000$ when cracking safe... You can easily earn almost 20.000$ with a single robbery (if you are fast), same for the other Didier Sachs shop in Rodeo (40.000$ total if you are lucky).
Let's move to the other robbery which is the Betting Shops, there are 2 Betting Shops in LS, 1 is located in Mulholland and 1 in Montgomery, in each shop you can get 4.000$ to 5.000$ from the cashier and 9.000$ to 13.000$ from cracking the safe with a payout of almost 22.000$ from each robbery.
After this go to Palomino Creek (LS) where you can find the Bank which gives you 3.000$ to 5.000$ from the cashier and 4.000$ to 8.000$ from cracking the safe.
Remember this: Whenever you rob the cashier and/or crack the safe you will not be able to rob again the same store until the cooldown ends which is 10 Minutes (realtime).
Tipo Spawn in LS Airport and get a Helicopter (hoping you have decent skills to drive it) and start your Money Rush robbing the Didier Sachs in LS Airport, then fly to the other one in Rodeo and rob it, then fly to Montgomery and rob the Betting Shop after BS fly to Palomino Creek to rob the bank and after that fly to the other Betting Shop in Mulholland, after this, I suggest you rob the Strip Club in East Los Santos (this robbery is good and will let you be able to gain some time until cooldown ends for other stores), do this again and again;
Try to avoid getting arrested and try to be a Threat so Cops will not kill you and have to be arrested.
P.S: I will continue writing this guide those days, still unfinished.

