A quick and easy $22k:
In the mission Out of Court Settlement issued by Martin Madrazo, the lawyer's car is worth $22k. How do I sell it, you may be wondering? You must first either steal the car from him by throwing him out then killing him, or killing him while he's moving. The reason for this is that you cannot pick up the evidence before taking the car. It will not work. Next, you take his car to your garage (it is recommended you memorize your house's/garage's position, instead of roaming around the city looking for it) and store it there. After that, you must go back and retrieve the evidence, and take it to Martin's house. The mission will be complete, and you will earn $4,500-$9,000 and some RP. Return to your garage and retrieve the car, take it to the nearest LSC, and sell it for a maximum of $22,000 (don't damage it for the full amount).
I made a ton of money by doing this.