That pretty much sums it up: some people don't want to even begin to deal with mods. Others are cautiously interested, depending on whether the mod was done by a "name" (Mitch Sugarman comes to mind for me), or whether the mod was very simple and obviously a benefit.
Also there are certain components that are very frequently modified and the nature of the mods has become fairly well known. I purchased a modified EAR 834 phono stage, compared it to my unmodified one, and was frankly blown away at how much better the modified one was (even that I loved the stock model). HOWEVER, the price I paid for the modified one probably had a premium of maybe one quarter or less the likely cost of the mod (getting around to your actual question!).
Art
Also there are certain components that are very frequently modified and the nature of the mods has become fairly well known. I purchased a modified EAR 834 phono stage, compared it to my unmodified one, and was frankly blown away at how much better the modified one was (even that I loved the stock model). HOWEVER, the price I paid for the modified one probably had a premium of maybe one quarter or less the likely cost of the mod (getting around to your actual question!).
Art