I found the master coin code for Xbox on the 24th and someone tried it on the PS2 yesterday and it works also. It will give you unlimited coins to use in the game.
Choose a vs. game and load a profile for each fighter. Make sure each has a nice amount of each coin. Wager one round as normal and then after the first fight has ONLY player one open the wager tag but not player 2. Have player one press left or right and they will hear a coin sound and then press up and you will hear coins being bet. The sound will stop and hit the wager button again to make the tag disappear. Now both fighters choose characters and fight with player 1 beating player 2. Player one will win money but Player 2 WILL NOT lose any. Within a hour you can rack up enough to open all coffins.
With the Xbox hit the right trigger to open the wager window and on the PS2 hit the R2 button.
When you open the wager tab with player one the coins are as follows to bet since you cant tell what coin you are betting.
Right once and up = Onyx
Right twice and up = Jade
Right three and up = Ruby
Right four and up = Gold
Right five and up = Platinum
Right six and up = Sapphire
Update 25 Nov. 3:02 PM EST: Thanks to our readers Matt Lynch and Scott Ramsey for testing this out on the Gamecube version. This trick does work on all three platforms.