I purchased a new Velbon Dolly a while ago for $50. http://www.velbon-tripod.com/accessories.htm (Dolly DL-11)
This is perhaps the cheapest dolly on the market, both in price and quality. In fact, the dolly is so cheap it's borderline unuseable. I don't know how anyone could get any use out of this dolly as is.
However I went to a local hardware store and purchased higher-end castors ($20 for 3) with bigger, softer rubber-type wheels. This made a huge difference. I went from having a $50 dolly that performed like a $15 dolly to a $70 dolly that performed like a $200 dolly. Huzzah!
A dolly like mine that rolls directly on the ground needs at least two things to work: 1- it needs an extremely flat, quiet, and even surface, and 2- it requires a camera-person AND a dolly person to push, pull, and guide the dolly. Thus, you are still extremely limited in its' usage.
A dolly on a track is head and shoulders better for stability and smoothness, but it is more demanding: you need more room, more set-up and tear-down time, bigger transportation requirements, far less discreet etc.
Good luck!
In Solidarity,
Alain
http://www.crazyredacesproductions.blogspot.com