Yeah, $500-$600 is a ridiculously low estimate. When things were hot a couple years ago, a nice clean 19" Dagor was bringing closer to $2K. The longer f/7.7 Dagors, 16.5" and longer, are just not as common as 14" and shorter.
They are not quite as sharp or as contrasty as some "newer" designs, but they are excellent "all-arounders" especially for big negatives for contact printing, exhibiting generous angle of coverage, good sharpness, fairly fast max aperture, and really decent contrast even in uncoated examples owing to the all-cemented design with a minimum of air/glass interfaces.