Talk to Minikin and buy a high damage weapon with a fast burst rate and large magazine, such as the Oblivion-Arc 1. Play the "Space Plumber" mission. Quickly run through the mission collecting all the money packs and killing the zombies. Sometimes the zombies will drop money. It takes approximately two minutes to complete the mission and collect all the money. You will get around $20k per run. Keep replying the mission and collecting the money as many times as desired.