This cheat takes a while but it's worth it. Once you have a truck successfully complete a mission, any mission is fine but the more money you earn on the mission the faster you'll gain money. After you have completed the mission. Save the game as you regularly would, then go to garage, profiles, save game, press B, then choose no to save game. This will put you back on the end of mission screen. After this don't save and keep doing it again, it'll work over and over.