After recruiting Zero, stop at his shop and complete all his missions. You will be awarded with a $ spot where you can periodically claim free money. You can also make more money at Zero's place by doing an extra mission (after you have already completed all the missions to get the shop as an asset). Go to the back room in the shop and step into the red marker to trigger a mission where you use the RC Baron to destroy Berkely's shipping vans. For each van you destroy, you will get money. There is a time limit for each time you play the mission. You can replay this mission as many times as desired.