Here's a guide that lists some ways you can make money in GTA 5.
Invest in a property - You can buy a property which will earn you money daily, or weekly. Check out our properties locations and list guide for more information.
Some properties also have missions that can earn you some money as well. For example if you purchase the McKenzie Field Hangar, you can do arms trafficking missions that will earn you some money.
Finding hidden collectibles - You can find various hidden collectibles in the game that will earn you money when you pick them up. Check out the GTA 5 section to find their locations.
Robbing stores - You can rob various stores around the map, some give more money than others. Check out our shop robberies guide and map for more information.
Security vans - You can destroy the rear doors of a security van (gruppe, shoot the handlebars to open the doors, or use explosives).
This will drop suitcases with money. Security vans can sometimes be found as random events near gas stations.
Robbing random pedestrians - Find an ATM and wait for people to use it, then rob them.
Perfecting the heists - When doing heists, always pick the most skilled people for driving, shooting, hacking, etc. this will ensure you have the highest chance at a good payout once you complete the heist.
Main Missions - Just progressing through the main missions will also make you some money.
Side Missions - Side missions like Strangers & Freaks, Random events, etc, can sometimes give you a big reward, so don't skip them. Taxi cab missions can also make you money.
Invest in the stock market - Check out the stock market money making guide to find out how to get rich using the stock market.
Saving Lester's assassination missions - Simply, if you save all of Lester's missions for after you complete the main storyline, you can make a lot more money.
See below for a rundown of what to invest in during those missions.
Stock Investment Money Making Advice (In order) Here's a small guide on how to make money with Lester's assasination missions for Franklin after you finish main storyline. Thanks to marauder for the guide. (Guide updated)
STEP 1 - BEFORE the second assassination mission invest in [DEB] Debonaire (LCN). Sell at 80% return.
STEP 2 - Invest in [RWC] Redwood (LCN). Advance in game clock 2 days. Sell at 300% return.
STEP 3 - BEFORE the third assassination mission invest in [FRT] Fruit (BAWSAQ). Sell at 50% return.
STEP 4 - AFTER the fourth assassination mission invest in [VAP] Vapid (BAWSAQ). Advance in game clock 2 days. Sell at 200% return. (Thanks to BuyBuySellSell in the comments for the fix)
STEP 5 - BEFORE the fifth assassination mission invest in [GCD] GoldCoast (LCN). Sell at 80% return.
If you're looking to make money in GTA Online, check out the Making Money in GTA Online article.