Money is always short at the start of GTA 5. Lester's assassination missions and the stock market can grow the funds of all three characters.
The Hotel Assassination early in the story has to be finished to continue the main story. The later assassination missions on the map (Vice, Multi Target, Bus, Construction) are better saved for later.
GTA 5 stock returns are a percentage of the money invested. With a small stake early on (for example $100,000), a 50% return is only $50,000.
Finish the last heist of the main story, The Big Score, and the payout after that last mission leaves Michael, Franklin, and Trevor each with tens of millions of dollars.
Snowball: start the stock work with $30 million per character, and a 50% rise adds $15 million at once. Reinvesting the profit grows the money quickly.