In total, there are 22 cash wads that need to be collected. It is pretty easy to gather these and not end up burning them. Most are on the floor, some underneath body parts. There's one on the upper level, and there's one in the main room on top of a glass case (that's the one that tripped me up for a while). It is possible to get this along with the other achievement, so I recommend doing that. As such, I would wait to count up all the cash until you have cleaned up the place, at least mostly.
Before you clock out, save the game. It tells you how much cash you amassed once you clock out, When I clocked out the first time, I only had 1.82 million, but I was only missing 1 wad, so I'm assuming that means that each model has its own value -- because the math works out to each one averaging 90k, but 2m - 90k isn't 1.82 million. So the non-wadded up bills are worth double? I wish I could help more in trying to figure out how many you are missing, if you clock out and happen to be missing any, but it's inconsistent.