... then why don't you put that payments into the game. Wouldn't it be great if you could spend it e.g. on better equipment?

E.g. like...
... a vakuum cleaner that's faster and / or makes less noise.
... some clothing that gives you half a second more reaction time from the cops when you step into their vision cones. Something very unsuspicious...
... better running clothes / shoes
... a one-time helper that distracts the cop(s) for a short while. Like e.g. a rocker that loudly drives from one side of the place to the other and let all cops turn their heads in the direction of the rocker for some seconds before he drives of and they return to their patrols
... something like a retirement fonds. Could be interessting to create achievements for the game based on how much you put into it. So the players would be motivated to invest not everything in shiny new stuff that makes levels easier and go more pro. I mean... some people like it a bit easier, those could spend the mobmoney on equipment. Others like it hardcore. Those could spend the money in the retirement fonds to increase their score. If you put an upper limit on the total of cash that can be earned you would reward them too by creating hard to earn achievements

Wouldn't that be great?