In the bottom left corner next to the "Profile" button is the Experience bar.
Experience is another way to get bulks of coins and upgrades to your rank. You earn experience points by winning online multiplayer games and single player missions. This is a great way of getting good coins for buying weapons. It also upgrades your Rank. When you upgrade your Rank, you get more coins. The higher the Rank, the more coins you get from each upgrade. The best way to get experience points is to either defeat wave after wave on Survival, or play Team Battles on the Multiplayer Mode over and over again to keep getting them continuously.