I bought a couple of the Mossbergs at Wally World a few yesrs back for $209 each. They still have them for around $250, cheaper than anyone else. The 870 and BPS are obvious steps up in quality. I wouldn't even bother with an old High Standard or Stevens, merely to save $50 or so, unless there's some sort of sentimental attraction. I don't think I've ever seen a decent Model 42 for less than $1k, and most are $1200-$1500 or more. Strictly a collector gun at that point, when a nice O/U from Browning or Beretta could be had in the $1500-$1800 range.
