one way to make easy money is to hijack expensive cars and take them to the Chop-Shop to sell. This though does involves waiting, you can avoid waiting however by cheating the system by setting the time on your phone in advance. You will have to do this often but is is worth it as it is an easy way of getting plenty of extra cash quick. When you set the time in advance 30 minutes you will manually reset the timer so you can sell more cars than just one at the time.
High Chop-Shop Valued Vehicles
Störer Level 1 : $270,000
Störer Level 2 : $405,000
Vanguard Level 3 : $688,000
Mc4 Firestorm : $918,000