Or save up for RARE parts. They get VERY expensive. They will appear in "New Game+" and "New Game++." Every time you beat the game you can start it over, more parts will appear. There will be a new NPC on the deck of the Duel Ship. There are 6 versions of each type of rare part. You will have access to the 5 and 6 versions in the second and third playthrough respectively.
You can also buy X versions of each Dahak type (R, G, C, and S) after you beat the game the first time (gives you extra combo hits, +stats, alt colors, etc). The Omega type is unlocked on your third playthrough (buy it for a cool 60K Rings)...
Whatever leftover money you have left after all that, burn it on magic cinematic ceramic plates (can't explain THAT!).