Seems you can find Parker Reproduction 20's in the $4-5000 range fairly easily, and except for the occasional lug failure they are touted to be better for extended shooting than their 100 year old counterparts. Point being, anything over that price and you are paying for collectibility rather than shootability.
Bingo... there it is. Long before I became interested in SxS double shotguns, original high condition Parkers were collectible and coveted by more shooters than any other vintage U.S. made double.
There was a limited supply of Parker small bore guns in very good condition, and more important, there was more than enough demand to sustain high prices that only seemed to keep increasing.
I have a 20 ga. L.C. Smith Ideal grade with auto ejectors in very nice condition. It is about the grade equivalent of a Parker GHE. The past, present, and probably future market dictates that a 20 ga Parker GHE in equal condition will always sell for quite a bit more than my 20 ga. Ideal grade. I could disagree with that, but i'm not going to change it anymore than I could change the flow of the Mississippi River by swimming upstream.