How can I increase my currency?

Money seems to get harder to get as you level up. And things start costing more. Raiding rebel camps only helps a little. Selling other resources when the helicopter arrives will get you more if you have upgraded the commercial hub (helo pad) a fair amount.

Changing class to a trader will give you a buff that increases the money generating speed.This helps a lot but means you lose the raider or farmer buffs if you prefer those.

Check through the Resource production research sections in the institute for Flexible regulation to also give your money generating a boost