Don't we already have two (annual or bi-annual) published price guides out there?,
Mr.Pinball and PinballEric guides
and at least two online pinball pricing websites/archives?
Bostonpinball and pinballprice.com
Those price guides and game sale price archives are all very subjective,
since a game's condition
greatly affects the games worth to a certain buyer.
And what someone calls near mint, might only be average condition to another.
If you can come up with a standard uniform condition grading system,
(Like that used for grading comics or coins)
that would go along way to defining what a game might be worth.
Condition on a pinball game,
should be broken down into several categories.
BackGlass or Translite
Playfield
Plastic Sets/Ramps/Bumper bodies
Cabinet & Legs
Innards (MPU boards sets for S/S pins or Relays/Stepper units, score motors etc for E/Ms)

Then define how much (% or sq.in.) of paint flaking on BG
or wear on PF
or gouges or scratches (number or length) on CAB
or Rusting on Legs
or warping or cracks of plastics
effects the grade or the condition of the game.
Then somehow total all that up and weight or average
those scores/grades into a total Game condition.
Until a uniform standard for grading condition is universally accepted,
any pricing guide values are fairly useless and moot.

Might want to add state or region game was bought,
as prices on west coast are generally higher than east coast, or Europe or Down under.