Unless it's a .32 or a 4 inch model, the SP's don't go for more than $400 used on FeeBroker.I dunno what GB sells them for, but I know at retail even used they dont hang around that
long. If there was a depression in pricing its probably because of covid barf factor, not the gun itself. That and I would expect to some degree all carry-type revolvers to be taking big hits in the past 1-3 years or so as (some) people dump them to move onto more normal things.
That's not to say the rest of the revolver listings beyond the collectible Smiths or Colts are having the bottom fall out for prices, older Ruger Six's are usually going for more than that, GP's too.