I have owned quite a few treadmills over the years and have learned a few things about what to buy and what not to buy. The qualities you are searching for in a treadmill, will be something along the lines of a quiet motor, a cushioned deck, a stable tread, motorized, etc., and they usually cost upwards of $1000 or more. There are cheaper treadmills out there , but you have to be careful of the quality and durability. Make sure it has a belt & motor warranty of at least 2 years. Try to find one with a running surface of at least 18 to 20" wide & 50 to 60" long. The more wear and tear you plan to put on one, the more quality you need. You don't want to have to be replacing one every year or so - been there done that!
I bought a treadmill once at a major dept. store chain for around $400, and within months, the tread would slide over to the side & I would have to manually roll it back to correct position, the belt would slip while I was running and it was literally falling apart around me, screws falling out, etc.
The treadmill I own now is a Reebok RX 4000 and I bought it used from a friend - 9 years ago and it is still going strong! So, you're best bet is to research online and visit some Specialty Fitness stores that specialize in exercise equipment. You might find some discount or closeout specials online too.
Here is a link for some treadmill reviews and info: http://www.treadmill-ratings-n-reviews.net/
Good luck in your search!!