After any mission in Free Roam mode, go to a jewelry store and break in. When inside, take the register and exit. Make a right and go straight until your base is on the right side. Go up the first set of stairs and stop. Turn towards the stairs leading farther up. Throw the register against the stairs in this position until it breaks. When it breaks open, stand over to collect the money. If done correctly, the register should keep giving you money until it reaches $999. Note: The money you collect may not be there during missions.