The installation of a recoil pad instantly removes the rifle from mainstream collector interest. If so, the most relevant questions are what amount of loss from "collector" and as noted, the effect of being "shooter". In the most common chambering, 30-06, probably suffers less collector value loss in terms of chambering. If the sub model is "Standard" or "Featherweight", that also contributing to lesser loss compared to such as a factory target configuration.
The 'net' is not typically a great loss in the condition described if factually any at all. The great righteous popularity of these guns for what they are intrinsically as a sporting arm, sustains a great amount of the market value.
Under the circumstances, given the reported condition, I'd not assess any material loss beyond the value hit if the stock is trimmed short LOP, or the pad is old and requiring replacement, the latter factor, cost of replacement. The former a substantial usability hit, functionally equating to the cost of a normal LOP recoil pad stock. I'd either address the aft sight in respect of an inexpensive flip up open sight or a dovetail blank if to be scoped anyway.
If all that doesn't confuse you... I've not adequately described your situation! Factual value nowadays, I leave to other folks more conversant with contemporary values. The only general observation the marketplace 'of things', is taking an overall inflationary hit indirectly. As reflecting steep upward trend relating to less available "disposable" purchaser cash!
Econ 101!
Congrats on a nice rifle!
Best!
John
