I get the big retailers w/ overhead and the wiggle room. I look there and say easy 30% off if I was going to a FTF sale or local Gun Shop, cash sale w/no trade.
I also do the Completed Search at most sites also keeping track of how long/often a particular shotguns be at the auction. Seems to make a good reference point to value. Again there are some that, from the production numbers found here, don't seem to warrant the starting price even in the condition they appear to be in.
Collecting or acquiring as in my case isn't necessarily going to be rational. I just got back the barrels from my 20ga Field Grade, with little to no case color left, cost with shipping both ways $250. Half what I paid for the gun 14 years ago. Not a uncommon gun by the numbers (30K+) Still less then another but to most it might have not been worth the amount to make that barrel perfect again. Also seeing the asking prices of those at auction with the dents in the barrels I then factor in that repair alone based on what I paid and say not worth the price "To Me".
This part may need to be its own thread but I'll try it here for a while and see. I was looking at a Doug Turnbull Restoration Catalog that showed L.C. Smiths, Parkers and others that were "Turned Into" Higher Grade guns then they started life out as. Field Grades to DeLux Grade, and others. Seeing what a full blown Restoration cost for Wood and Metal on a Field Grade one could never get their money back. I see Cabela's with a Turnbull Restoration Trap Grade for less then the cost of a restoration of a Field Grade. Traps in 12ga numbered <2700 which seems pretty rare? Did the Restoration Hurt the value of this since its rarity is what was going to give it its value to begin with?