That's going to be a tough one...
Generally with that zoom range quality and speed will cost you...also, you won't be able to use many of the cheaper EF-S lenses since they're designed for crop bodies, not full frame...
The problem is that since all current Canon DSLRs use EF mounts, the prices on EF lenses haven't dropped, even if they're older, since they can be used on anything made in the past 25 or so years.
If I were you, I'd look into some third party (Sigma, Tamron mostly) lenses...maybe you can pick up a cheap 18-200 Tamron or something...again, it won't be fast and you won't get L quality images and you'll likely be unable to use the wide end...but that's all I can think of that would fit in your budget...