If you limit spending, be smart, and put in work, you will watch your bank account soar quickly. Dint worry about having a bunch of super cars and having every vehicle possible, buy two very reliable cars (fast 2 door & fast 4 door), maybe a motorcycle, the vehicles your crew requires, and leave it at that for now. Put away anything you can for now and then once you have a comfortable amount saved up, you can spoil yourself a little at a time.
Yeah. I have made some purchases but nothing crazy, except the Roosevelt which I just loved. Besides that I've only got vehicles I got for free (barring the Seashark). I don't have a super car and the only somewhat speedy car I have now is the free Elegy. Put bulletproof tires on it and it's good enough for lots of missions!
I would like to eventually buy the buzzard and rhino, though. The chopper first because like all choppers it can carry four people and it's quite fast, at the very least rivalling the cheaper airplanes. Considering the fact it's armed and it can take-off and land vertically I figure I won't buy a plane until I've saved up for the higher carry capacity ones. Or the vestra.