Many different ways ill list the ways of looting Easy, Medium,Hard and extreme
Easy: not the most efficient way is to loot militarized zones including NWAF,NEAF,Balota and any other place now this is not the best method but if your a solo player looking to make money you can easy walk off with 20-30k+ worth of gear in a car.
Medium: This is looting radiation zone, there are not many people that go to this area you are required to have a gas mask from the black market. with a helicopter you will average 30-40k+
Medium: Doing missions(Not checkpoints) if you're the first person to a mission and you very fast with completing it you can walk off with a box full of guns, money and other supplies. this can average out to 50k-70k (Make sure to take the cash out of the crate and if possible grab money from AI)
Hard: Capturing checkpoints it takes 10 minutes when the zone is clear to capture this mission, when you're doing this mission other people can see if someone is in the zone and come to kill you also after a while more AI will spawn in and try to hunt you down. average on this crate is 50k-80k (grab money from crates and dead AI)
Hard: Raiding bases this one is random depending on who you raid, if a person has max bank they need a place to store their cash, so whatever they get after max they will store in their base a long with guns,supplies,raiding supplies,cash vehicles and flag pole. Now this is all a random amount i cannot say how much you will get off a raid.
Extreme: CDF headquarters this one will give you a lot of stuff there is building crates,explosive crates, weapon crates and more you will also get money off of dead body there is 40+ Ai on this island so you will need your willfulness and maybe a team to speed things up because some Ai are hidden they are very sneaky .
This is my thinking on money making... hopefully you get mills good luck
this is how your bank will be looking after my guide