Maybe about $500 tops? Preferably Tube!
In that price range, for a tube amp I'd look for a used Peavey Classic 30 or Delta Blues, or a Fender Blues Jr. A Peavey Delta Blues 2x10 with the speakers replaced with some 10" Jensen's is a revelation!
A solid state amp with great tone that I'm really loving is the recently released Roland Jazz Chorus 40. Lots of tonal variety, and a redesigned Distortion that works quite well for lower gain grit - sometimes I play it with the Distortion on around 11 o'clock for just a little hair on the tone. They MAP for around $599, but I got mine brand new for considerably less than that, and there might be a store model or return from MF or GC that you could pick up for 5 bills.
I also agree with unclrob - if you can stretch your budget a little, a Fender Princeton Reverb Reissue (used ones are out there) is about the best amp to use for building a tone palette.