About the most affordable, and good ones, are Hobarts, made in the USA. Look at their 124 or 140 series. That represents how many amps they peak at. I've got a 125 at home and it's great.
Of course anything with Lincoln's name on it is good too. If you find them on sale you might be able to get one for the same price as a Hobart, I don't know.
I've heard some pretty good things about Hobarts and I plan on buying one once I get all the crap I want to do to the stang done and do a little bit mroe research.