If you don't have to go over 35 MPH, get the 50cc models. They are registered and insured as mopeds and cost between 1K and 2K.
I bought a used Yamaha Zuma for $800 and it runs like a top.
Greg White (Two Wheel Tuesday host on SpeedTV) rode a Zuma cross-country. I think he had to have the engine rebuilt half-way through because he was riding with throttle entirely open the whole time.
http://www.2wf.com/articles/news/8BD372D2-441B-4DEE-925C-5BF10F47F77E.asp