At the end of level 3-2 when you are on the dock and the refinery is about to explode, turn around and run to where Irving's boat was and open the case for 3,000 gold

Head in the direction you are supposed and defeat the Big shirtless guy with the dogs for a 5,000 ring.

If you die after that, you keep that money and can upgrade your weapons with any money you find during those 2 minutes.

After upgrading, you will respawn at the checkpoint and be able to do it all again.

This might be time consuming but it is easy money.