Fight the Boss of Hourglass Caves. He is easy, and can be defeated in under a minute. Kill him, and get as close to the location he was last at when he died. Now you can collect all of the money that comes out of him.
If you do this several times, you soon accumulate a serious stash of dosh.
Also, if you a 'rewind' immediately after you have collecting all of the coins after defeating a boss a lot of the coins will return allowing you to pick up more!