I'm fairly new to the forums and so haven't been privy to a lot of strategies, but it seems like there is actually very little need for an economy in this game. If you just rob/attack other players to the point where you can eventually farm M4s it seems like the money from both farming and attacking would be enough to purchase all the SGs you would need (from what I see the only item people spend money on). Then with your RPs you just dump them into NVGs and Excelleros.
MIRITE?