I just looked - one can buy a Superliner 21" K-Line coach for maybe $220 plus $17 shipping. I don't know how often they show up, and yes I know that folks offer them for $600 each. But the one I spotted was fixed price.
That is not much more than K-Line's original price. I doubt seriously that such fine products could be produced today for that kind of price. If I wanted a Superliner, I assure you I would be picking these off at that price.
I have maybe 20 K-Line 21" cars. They were top of the line at the time. GGDs are better, but so is the price. Opinion.