How can I loot more cash on my APC?

From what I understand the amount you get from each raid will depend on the load capacity of your APC. When you win you will raid resources (and money if you are using the raider class APC only) with an amount related to the load capacity of your APC. This will also depend on what resources and money the target has.

If they have lots of food then you will get a lot of that, likewise with other resources. You can scout first to see what they have. You will only get lots of money if they have little other resources.

The best way to increase the return is to increase your load capacity. Use vehicles as much as possible and upgrade etc to add more troops, this will help.