Your treadmill price will depend on the brand. Well known treadmill brands like Proform, Nordictrack, and Cybex tend to be a bit pricier than other brands, but they make up for it in quality and in durability. I have had my Proform Crosswalk since 1993 - and it is still in excellent shape. Even the best price treadmill doesn't come cheap. Be willing to invest a good quality treadmill. You'll have it for years to come.
If you want to lose weight quickly, exercise is the way to go. But before implementing any exercise program, you must visit your doctor. Explain to him or her that you are looking to lose weight. I'm sure your doctor will recommend the best aerobic exercise which you can later tailor to your needs