Hi GetSmart! It really depends on what your budget IS, I'm afraid. A very good rule of thumb is to avoid the treadmills in the under $1000 price range (new) - they rarely stand up to a lot of use.
This is, unfortunately, the time of year when manufacturers HOPE you'll be buying (ahhh, those New Year's Resolutions!
) so you won't be looking at bargains right now. If you do your research on the models that people might be selling on, though, you might get lucky second-hand. Remember that it is an investment though, so not having a warranty might be a deal-breaker for you - it depends on what features are important to YOU.
I bought mine from Smooth Fitness and I ADORE it. Yes, it cost us quite a bit, but we got it on sale 40% off, and it has an amazing warranty and loads of features. I certainly looked at many that were twice the price with a lesser warranty and lesser features.
There are lots out there to choose from - do a Google search for 'best treadmills' and you'll call up reviews
That's a good starting point.
To state the obvious - if it seems to good to be true (especially second-hand) it almost certainly is. Good luck!