Big Money is the fifth and penultimate mission in the Stones Unturned DLC. The mission starts as you and Donovan make your way to the crash site of an important plane and the bulk of the mission is you making your way on foot to the plane and dispatching the enemies on the way (and there's a lot of them). You'll then have a cutscene where you find out the payload you were looking for has already been taken out of the plane. Right after that, you'll be greeted by a new batch of enemies including a turret up a hill. Make intensive use of cover, peek with caution and kill everyone.
Once you've secured the area, the mission will be over and you'll have to get on the same vehicle as the one from the wild car chase at the end of Devotchka.
WARNING : If you're like me, the achievement was scarily delayed and will pop while you're being driven by Donovan, just before the first firefight of the "There are no Dominos" mission.