This is the best way to earn at least 10,000G by just finishing one stage:

Finish Story mode
Buy 2 G300 Rays as your primary and secondary weapons
Buy 6 Spare Ammo Upgrades (3 for each laser gun)
Go to Stage one, finish the stage without using the G300
While battling the Retrievers, collect as many Space Controls as possible, then use a weapon (SAM Missile Launcher Recommended) to end the level