1 - Crane
After finished the game, go to the crane room (Door above the first door inside the tower on the ship.) There will be a giant stash of money. Small amounts of money spawn here again eventually in increments of about 350.
2 - Gates (This is the best way for sure)
Inside the ship tower, first door, there is a room with Auric, and Dread-Beard. In the middle, there is a hall blocked by elemental gates. This is basically the same thing as the block tower. It is best to be used after you finish the game because you will get about 1350 each time. Once you clear the gates with the proper elements and get your money, leave the room go back in, and do it again! This gives unlimited money.
3 - Arenas
The higher ranked arenas (Such as one point health), gives about 2100 each time you complete it.
EXP:
1 and only - Simple...
Pretty simple, set the difficulty to easy and do the higher chapters. Levels are extremely easy to accomplish.