1: Get cheap social club accounts with gta, you can either use combos and social club checker or just buy cheap accounts ( needs to be pc)
2: Use a ranking service to get the accounts with huge amounts of cash and level. It costs around 5$-10$ to put 1 billion gta $
3: Sell the account for 15$ to 20$, use places like eldorado.gg ; playerup; g2a to sell the accounts 
5 Repeat it all over again and boom easy 500 to 1k per month