Mortal Kombat: Deadly Alliance

Create two player profiles and earn some coins with each one. Then start a vs. match and wager some coins (R button).
After that only player 1 bids coins, wins the match and earns the coins but player 2 won't loose any.
You can stop when you have about 30000 - 40000 coins of each color, you won't need more to unlock all the krypt secrets.

You won't see what you bid so here's what you need to press for the desired color:
right, up = Onyx
right x2, up = Jade
right x3, up = Ruby
right x4, up = Gold
right x5, up = Platinum
right x6, up = Saphire

Go into the Krypt and go to the koffin listed and buy, you will unlock that costume.

Example: AA (Koffin number) 343 (Amount Cost) Gold (What resource you use)

Arena: Koffin - Price

Moloch's Lair: ZD - 98 Ruby Koins

Nethership: DS - 1472 Ruby Koins

Palace Grounds: KR - 4222 Sapphire Koins

Dragonfly: WT - 1400 Jade Koins

House of Pekara: HP - 2093 Onyx Koins

Sarna Ruins: PW - 2006 Sapphire Koins

Lava Shrine: II - 1843 Gold Koins

Character (Price to Unlock) - Which Koffin

Cyrax (3003 Platinum Koins) - CN
Drahmin (6500 Sapphire Koins) - UR
Frost (208 Ruby Koins) - IV
Hsu Hao (3317 Jade Koins) - MW
Jax (3780 Ruby Koins) - SA
Kitana (2931 Sapphire) - KI
Nitara (4022 Gold Koins) - TI
Raiden (3116 Jade Koins) - XG
Reptile (3822 Gold Koins) - LL