asking "how cheap can I get a lens" is the same as asking "how crappy can my pictures be?"
However, there are a few gems to be had in the lower dollar-vale range. The stand-out "good" lens in the "cheap" range is the Canon 50mm F/1.8 II. You'll see it referred to as the "nifty fifty", "thrifty fifty", or "plastic fantastic".
It is a "prime" lens, you zoom in and out by moving closer and further form your subject. This allows simpler construction so the glass is pretty good for a $125 lens. You can get it for just over a hundred bucks refurbished from Canon right now. They have a year warranty and I have been very happy with their "refurbished" units.
If you go to Canon's refurb lens page and sort by cheapness
, I have the second lens in line too. It isn't exactly "sharp", but it does have some reach. With your camera body it would give you a 480mm effective focal length (thats very zoomed in).
Here's a picture from that lens zoomed out, then in, then cropped to 100% zoomed in.



You will find that there are basically no shortcuts in photography gear. From lenses to tripods, you get what you pay for.
75-300 lens at f/5.6 (about $100)
300mm F/4 non-IS at f/5.6 (about $750)