Play the Panic Room mission until you get an objective called "Find the Panic Room".
However instead of trying to complete the objective, you need to head to the reception area on the ground floor, i.e. don't go into the panic room!
You need to jump onto an object and then boost from there to look at the ceiling. You'll find lots of money that are not visible from the ground, but can be collected provided you're on top of an objected (boosted up).
Collect all the cash while standing on the objects, and then just restart the mission and rinse and repeat! Easy money.
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