A good place to farm gold is by the jackpot 3 screens left of the boss. You can go down a screen and then immediately go back up and it'll lure 2 mobs to you that drop a lot of money. Once you get good you can even do the process twice real quick before fighting any of the mobs so you can fight 4 at once.
For more info see this video (not mine), timestamp 39:00 -