You can re-play any level you've already beaten, and if you earn more money (by not leaving witnesses, low notoriety, etc.), the difference is added to your account balance (e.g. if you made $400,000 the first time and $500,000 the second time, then $100,000 is added to your balance). However, there is a glitch in the game that will let you add this difference to your account as many times as you want.

First, complete a level as poorly as possible, earning as little money as you can. Then save the game and go back to that level. This time, earn as much money as you can, and make sure you create a save point immediately before escaping the level. After you escape, the mission review screens will show your new account balance. You can press the B button to return to the first of these screens, and then press they Y button to replay the level. A warning message pops up alerting you that your progress will be lost; however, it turns out that your save points and account balance are NOT reset. As soon as the level restarts, you can load the save point you created earlier and escape the level again. Your account balance will again be increased by the difference you earned. You can repeat this process as many times as you wish.