Vector is known for producing various low-cost products, many of which are intended for car usage. The VEC249S is an inexpensive low power car vacuum that can easily be found for under $10 in a lot of places.
This is a no-frills vacuum cleaner. It doesn't have any attachments or cleaning tools, it's just the vacuum and that's it. The cord is an appropriate length to clean a four-door sedan, and can be stored in the back end of the vacuum. It hooks into the cigarette lighter in your car for power.
The suction power of the vacuum is not very strong. Don't confuse this with a standard bag vacuum or one of the vacuums you find at a car wash place. It will pick up small rocks and various dust, but this is not something you'd vacuum your car out with prior to selling it. For general usage, just to keep your car tidy, this vacuum is good.
When you open it to clean it out, the cup stays secured in place as it's a bit rubbery, so there's no worry about accidentally spilling the dirt on yourself when cleaning it out, which is a big plus. Empty the cup, shake it a bit and you're good to go again. It doesn't hold a lot, but it should be more than enough to clean out the floorboards of the seats for most people without having to clean it out more than once.
If you need a vacuum for your car and don't really care about having your car showroom-nice, this vacuum is fine and definitely affordable.