In Chapter 4-2 you have to fight 2 Gigantes ( the big trolls) and they each have 15,000 pesetas on them. Supposedly if you want the 30,000 you have to kill them both the old-fashioned way, but instead just drop one of them into the magma pit and kill the other one, then after the fight collect the money from the one you fought, go back a room and then return to the boiler room to find 15,000 pesetas right on top of the magma trap.