I just bought a used 20/20 for $500 shipped. It is the latest black faced model. If you are buying the older chrome face I wouldn't pay more than $400 for it. You should also factor in the cost of a retube as most people tend not to retube unless a tube fails. EL84's are not as rugged as 6L6's and should be replaced, at a minimum, once a year with continuous use and not a lot of gigging.
$500 seems to be average for the blackfaced models. The models with the chrome knobs and the black face seem to be the newest and, of course, the price reflects this.
