There's a GTA5 thread this may be merged with.
Missions. There are a few that get you good sums of cash, but a lot of them you have to be a certain rank. One mission people do repeatedly is Coveted. You have to have a rank of 37 I think. 12k in a few minutes.
Survival mode/missions get you 20k for completion and are good ways to pick up weapons/ ammo, though if you die after, you lose weapons you picked up if you haven't bought them.
Its been awhile since I played. A Google search will probably help you better than here.