When you first start the game and have no money, you usually pick up somewhere from like 5 to 10 dollars. Then later in the game you start picking up hundreds of dollars. It seems to be that the more money that you have, the more that you will loot. Is this really how it works? Or is it your play time, missions completed, level, or something else?
Depends on the areas until playthrough 2.5 where everything is based on level 50 anyway.
Tip: The best way to earn money is to just sell all the guns you see that aren't any good. Quickly max out the backpack to make this easier. By doing this you will never need money, it is really not a limiting factor.
As a lvl 50 you can make millions by selling an hours normal loot.