How much depends on what you want, and how much you can afford for future maintenance and upkeep.
Twice in the past I bought used Jeep Cherokees for about $5,000, and put about $2,000 into them (new tires, new headliners, fix AC, some other tweaks). So, for about $7,000 I had good, dependable transportation.
The only reason I had to buy the second Jeep was because some other person ran into me and totaled the first one.
My first car I bought (and taught myself to drive) was a classic VW bug, convertible, for $850. I put another $100 into paint and body work, and had a perfect running car (manual stick). Of course, that was 1973, and you're not going to get a price like that now. 
Sadly, it's harder to find "basic" transportation cars anymore. Everything has added features which increase the original cost and increase maintenance and repair costs. Even if they don't need as many repairs, when they do need repairs it costs a lot more for parts and labor. Gone are the days when you could go to the junk yard for parts and do your own labor.
TCS found his work van on eBay, and it was the best bargain (and long running--still using it) ever. We found my first Jeep at a local used car dealership, and my second Jeep thru Craig's List. TCS bought his used trike thru Craig's List, and he is extremely happy with that purchase. Other used vehicles we bought in the past from people we knew.
Like Jane posted, have a knowledgeable mechanic check out the car.
