Easy 999 Coins
On your second time through the game, go to Simian Citadel and track down the magician monkey, Tommy, who hangs out by the fountain. Don't net him! Instead, let him summon his mouse thieves, and let them steal some of your money. When you hit them and they drop your coins, there will often be enough floating around to trigger the multipliers, turning the single coins that they stole into x 5 and x 10 coins. Grab them and you'll have way more money than you started with. Since Tommy will always supply more thieves, you can do this over and over until you've maxed out your coinage.
Play as Spike
Successfully complete the game. Then, highlight the "New Game" option on the main menu, hold L1 and press Start to play as Spike from Ape Escape.
Easy 999 Coins...