Just before continuing with the assassination missions, players can invest in the stocks of the company owned by the person they are targeting and cash out quickly when the mission ends. Smart players can take advantage of assassinations to invest in stocks and earn money. In these missions, players can rob jewellery stores and assassinate targets to earn vast sums of money.
These are two of the most interesting ways to earn money in the game, though you will have to wait until you unlock Lester's missions. Do note that this will raise your wanted level with the LSPD. If you wish to get a substantial amount, then look out for the armored trucks around Los Santos which players can rob. Moreover, NPC using the ATMs can be robbed. Players can rob 24/7 stores and liquor stores for easy money. Robbing people, vehicles and stores is one of the easiest ways to make a quick buck in GTA V story mode.
Here's how you can earn money and get rich in GTA V.