There is an infinite money loop in level 7. As you climb the rocket launch silo, there will be countdowns at various points, where you have to reach the next airlock (or whatever) before time runs out. If you let that timer run out, you simply restart the stage. You don't die, you don't continue, it just loops. The kicker is that you keep any points you gained, so you can easily milk this until you hit 500k.
The frustrating part is that the level only yields about 8000 points, so doing this is going to be a boring grind for like an hour if you're far off, but at least it's less boring than starting over and trying the whole game all over again.
Remember to smash boxes to get hidden cartridges or coins, as those do add points, and that the final countdown is after the two elecricity barriers. There's a platform right below an airlock and a 4 second countdown starts when you reach it. Just stand on that platform and wait the 4 seconds to enter the loop.