[PropellerHead]

As a general rule I value good condition used parts at ~50% of new. This applies if they are presently available to buy new. NLA parts, rare pieces or special performance bits notwithstanding.

New-In-Box but not from original source (i.e. no warranty, etc but still available from authorized source) I typically value at a 25% discount from new.

Items which are well used and worn, needing restoration (minor mechanical or cosmetic) are 'cores' and worth maybe 25% of new. Items of 'salvage' status good only for parts are a crap shoot.

I think this is valid. Since we don't see many PMO's available used (let alone as 'cores), I'll use a Weber as the closest thing:
New PMO ~$4k - the closest set of 'cores' would be servicable Webers which go for $1000 (+/-) if they are complete and not in need of complete re-work (new plating, bushings, etc - if it needs that, it iw worth considerably less on the open market).
100% webers, used but freshly rebuilt are perhaps $1800 for the set.

Frankly I wouldn't pay more for used PMO's over freshly rebuilt Webers. But that's just me. The Webers probably carry a slight premium for vintage car owners and the PMO's to racers.