Treadmill motors are usually cheap, but need the speed stepped down through a jackshaft arrangement (which isn't hard but needs to be planned out). They're usually pretty high speed and a higher than needed horsepower but are fairly cheap as used or surplus ones. If you can find an older low HP one they're usually a real bargain (I bought three single HP ones from a surplus suppler a few years ago for 20 bucks a piece). Stay away from the low voltage DC motors that need a step down transformer or special controller for the lower voltage.
Look for a cheap Cycletrol 150 controller on Ebay, they're my preference and work very well to control both speed and torque. Sometimes you can find a complete enclosed controller with everything assembled and just needing hookup, but just the controller board is often available cheaply and it is really easy to hook it up and use whatever control configuration you want (such as the controller being remote with only the on off switch and speed control wherever you find them convenient. The needed on/off switch and a potentiometer for the speed are cheap from Radio Shack.)
Keep an eye on Ebay for an auction where they are gonig cheap, I got two at different times for less than 50 bucks total each.
Treadmill controllers will also work, a bit more complicated for the hook up usually and they almost always require turning the speed to zero and then setting it back up after shutting the motor off which is inconvenient (I suppose that is to keep the treadmill from suddenly starting up full speed while you are standing on it). Treadmill motors and controllers can be had really cheaply if you buy an old treadmill, or maybe get it free, from someone needing to get rid of it and remove the parts from it yourself (which also gives you some belts, bearings and such to work with).
There are a number of other good DC controllers, the Cycletrol is just my preference.
