Bethany, If you mean a manual treadmill? I used to have one a few years ago and it was hard to coordinate and use comfotably. I would NEVER get one again. If you can afford to, get one with a motor. They work much better. There are lots of cheaper versions around now... you can pay anything from £200 all the way up to £5,000.
First thing to do is sit down with pen and paper and work out exactly what you want it to do? Is it for walking only? Do you want to run too? Will you be likely to need it for fast running? Work out the sort of speed you'll need it to do. The cheaper ones have a lower top speed. Also how big do you want the jumps in speed? Some increase the speed by fifths of a kmh whereas other only increase by full kmh.
Have you ever used a one in a gym? If not, I suggest you look round your local gym's. Some allow a free trial with introduction to the equipment. This would let you get a feel for speeds and how to safely use them. Will help you decide which would suit you best.
Mine was bought for walking only... I have a hip replacment and running will shorten the life of the false joint... it works great for what I want.... and goes much faster than I'll ever need. Cost me £250. I love it !
BTW any walking is good toning exercise for hips/thighs/legs/butt. If you have someone to go with you it's nice to walk round local parks and that's free!