Cash is the in-game currency that is used to buy weapons and equipment. It can be earned by harvesting animals or completing missions and challenges. Certain factors come into play when determining the amount of money received for harvesting an animal. Integrity and the animals rating affect the amount of cash one may receive.
The best way to earn cash in this game is to Canada Goose hunt. For this you will need the Wild Goose Chase Gear DLC For access to goose decoys and a proper Groundblinds. If you do not have this DLC, then you can go to a multiplayer server and hunt puma or Mountain Lion. They are a class 5 animal, so you can use the starter weapon, Ranger .243 preferably with .243 Polymer-Tip Bullet. They each give between 1000-1700 cash per harvest if you pass the harvest check. Turkey give 1000-1300 cash, and are easy and fun to hunt.