I know it really comes down to what a buyer is willing to pay
I think it's very important to focus on exactly this part right here.
It seems to me that the best way to get the answers you seek is to ask the folks who do it most often... maybe including some of the people who hang out in forums like this one. And it's hard to put numbers down on a gun unless and until we have an idea HOW the number is going to be used.
What I mean to say specifically is...
I believe it's a radically different number depending on any/all of these scenarios:
--estimate for insurance purposes? Replacement or collector "value" ?
--estimate if you wanted to SELL it?
----sell it to whom: end owner? Or quick sale to a dealer?
--estimate if you were a buyer seeking precisely that?
And the numbers returned hinge on the above thing
FAR more for some guns than for others. Example... Glock 19. Doesn't matter quite so much with the above. But a pre-64 Winchester Model 70? A heap!
On top of all that is
condition. To put it clearly, there are many folks who don't have half a clue how to properly denote or rate condition. And I don't mean to say they gloss over deficits with some bad intention but in most cases, they simply have no clue how major a deficit can be that they haven't noticed. Again I say, if you're talking about finish wear on a Glock 19, pffffft. If you're talking about a scratch in to the metal on a Registered Magnum, you're potentially talking about a significant percentage drop in condition and "value."
Keep in mind that I am truly trying to be helpful and not adversarial--
Folks who are active in this hobby get approached often with "what is this worth?!" and it's almost like approaching a doctor at a cocktail party and saying
"my brother in law has a pain in his abdomen -- whaddya think that is?!"