Resources Points are easy to take over and bring a solid income each click/round. And they mostly dont start a counterattack after you get it. Money convoys and bank robbery brings 5k each. Not sure which mission type from Petros tho. (For Bank robbery you have to be near a big town like Kavala in Altis). Most of the Missions bring some money but i would start with resources points first to have passive income while doing stuff. Factories bring money too.. But yeah - time brings money the most. You can also sell vehicles. So make sure to tow repair and fuel trucks back to HQ if you are in need Hope that helps - good luck :)