1. Have a party than is no higher than 4 people on a farming server, any higher makes the risk of somebody dying.
2. Weapons that are recommended for farming at your rank are: M16+ Barrel Mag, Barret 50 cal, Flamethrower+ Napalm, RPG, and Chainsaw.
3. Having items such as a pipe bomb or clock bomb can really help later round to rack up some cash, molotovs are also a great item to kill edgars and AI zombies.
4. Don't play Vs. Environment, play Arena. Arena gives a cash reward, an item, and fully heals you at the end of each wave. Arena also have people start off as zombies so you don't have to wait 5 minutes just to start getting good cash.