If you play your cards right, all three protagonists will be walking around with millions. However, the stock market is also the solution to the money problems you might have in single player. It would skew the balance and negatively impact the game experience of players who don't want to resort to using cheats. This means that players who use cheats to add large sums of money would be able to affect stock prices for non-cheating players as well, and Rockstar can't have that.
In single player there are GTA 5 cheats for pretty much everything, from making yourself invincible to maxing out your health and armour, but there isn't one that'll give you an infinite bank balance. While there is a ton of GTA 5 cheats for vehicles, weapons, invincibility, etc, there is no cheat code for infinite money.
There is no GTA 5 money cheat in story mode.
