Cheapest is probably the SW stuff.
I have a few fliers

- though these may all be too wide for your needs.
Clone - Gunship, Arc 170
Droid - TriFighter, Vulture
After this there's the ISP Speeder (can be expensive) and as already mentioned the Flash speeder (usually cheap), the Skiff (hard to find) and the Rebel Troop Cart (dirt cheap).
If you change your mind about cardstock - try
here.
If you are looking for a one/two man/alien vehicle try the old VOID range - still stocked by Scotia Grendel - the Syntha Tornado 2000 Grav Bike or one of the Cyclops tanks may work.