Currency

 Currency can be obtained in multiple ways, such as these options: 

Factories

You can earn money by setting up a factory and claiming it's contents. Factories can be leveled up by using components.

Missions

You can earn currency by taking part on a mission, they start randomly and are announced in the chat 1 minute before they start. High risk, high reward. You can either earn a useful gun in a supply drop (Ammo Drop) or an attack helicopter and even a tank that you can sell for money or keep it (highly doubt that you will sell a tank to be honest).

Scrap Dealer

The scrap dealer is a useful NPC, as many items can be sold there, and give money and RP points.

PvP

PvP is really useful in situations, you can earn alot of stuff there and also some weapons you don't normally get access to while trying to be a pacifist (that is if you don't have the RP level) High risk, high reward sometimes.