Seems to me that the main problem on determining the value of a particular machine is in "grading" the condition of the particular machine and the value of added mods.
Reminds me of the discussion when buying a used car.
The same situation exists with purchase of rare coins.
People become experts at grading those coins to determine the value and they even get a title and get paid for being a numismatic.
Companies are founded on that principal and some of those coins that are professionally evaluated are desired by many people, due to the established value that comes from these experts that is universally accepted.
And just like when you buy a used car, people don't want to pay for the undercoat or floor mats that are added by a dealer.
They search for the dealer that will give them those "adders" for free.
The value is often in the eye of the beholder.