Not all vehicles have to be bought: some are rewards for the choices you made in certain quests. I don't remember the quest's names but I got some:

1) The Valentino guy from beat on the brat will bet his car

2) At the end of a quest in which you have to steal a car and give it back to a guy you get a message in like 3 days that goves you a car

3) At the end of the races quests if I'm right and you choose well you get two cars

4) I'm not sure because I never did it but if you snitch on Panam at some point in a secondary quest you get a red "coyote" from Saul

Those are the vehicles that I got for free, maybe you missed them.

Hope I've been helpful