Step 1: Complete level 1 with a low score
Play the first level through holding the gun 'gangsta style' (sideways), shoot legs whenever you can, and don't worry about dieing. In fact, while fighting the boss, make sure to reduce your score below 20k by dieing. Note: Each time you die, your score gets reduced by half in order to continue.
$1337 for holding it gangsta style while killing
$500 for completing the level
$250 for 70 leg shots
Step 2: Gorgasms and completing levels with high scores
Buy the pump shotgun and upgrade the clip size. You may want to upgrade it to silver when you can to make it easier to start getting gorgasms to achieve high scores on levels to get a lot of cash... not to mention the $700 for completing each level with a new high score. Note: It is actually easier to sustain gorgasms without Extra Mutants because you need time to reload and the shotgun has a limited number of shots per clip.
Step 3: Replay the first level for a (slightly higher) high score
Then, if you want to make more money, replay the first level until you just barely achieve a new high score. Repeat ad nauseum for $200 each time. I suggest you write down your current high score, because I don't think there is a way to check it in the middle of playing. For example, I replayed the first level 28 times to make $5,600 and achieved the following "high scores" in this order:
With either shotgun, it is easy to get gorgasms and achieve really high scores because you get +1000 score for EACH kill! Feel free to go for the auto-shotgun first or use the highscore method on another level besides the first.