Also don't get duped by peak horsepower claims. The continuous duty power amount is the most important number, so make sure you don't purchase a machine before finding this out.
The truth is, the less power a machine has, the less time it will work for, and it will give out soon after. Most of the cheap models simply don't have the horsepower to stand up to faster and longer workouts.
Also, make sure you think about how fast the cheap treadmill can go. Most are either designed for walking or running, but not either/or. Therefore, consider which exercise you plan on doing the most of, and make your purchasing decision accordingly.