How much "Value" would you all add to a gun if it came in it's makers case that was canvas? Or if it was leather? Or, if it was oak and leather?
I take kind of a mixed view on this issue. Basically, the gun and case should "go together." A lower OQ and/or lower CC gun in a high condition O&L case is sorta like caramel sauce on sauerkraut --- some things just don't go. A higher OQ grade and /or higher CC gun in a ratty deal (canvas over softwood) case equally doesn't go. The higher the OQ grade the higher the market's expectation that it will come with suitable case.
Usable cases seem to range from about $100 to $800. If the gun comes with a case of higher value than fits to the gun, you could expect to pay the difference in retail value of the case. If the gun comes with a case of value less than you can reasonably expect, you may give it a ding equal to the difference in retail value.
Just about every auction has a plethora of cases. Track the values for a few auctions and you will develop a good idea of case values.
DDA