Depending on how much you plan to use it should dictate what you are willing to spend. Consumer reports rated TRUE, LANDICE and PRECOR, the highest, but these are serious treadmills upwards of $2,000. I would suggest your local Pennysaver or Bargain news type of place to try and pick up a used one. You are still better off buying a used really good machine than some of the cheaper machines brand new. We just picked up a 3 year old Landice for under $500 using this approach.
I would also steer clear from models that fold up, a good treadmill will weigh somewhere near 300-350 pounds so a fold up one will likely have compromised build materials to save weight.