Just a guess but
probably maybe north of $300
Check
this auction when it closes in a couple hours
This one closely reflects the condition of my pistol, probably an accurate market value. Sorry for the clumsy link.
41
https://www.ebay.com/itm/Vintage-Sheridan-E-Series-Co2-Air-Pistol-20-Cal-With-Original-Box-And-Paperwork-/163044850194?nma=true&si=bi8CBVW9BMtewxyauxM%252BvxsqbEQ%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557