used aircraft valuation

I have often heard (from people in the business) that a good opening offer
for what

(well) used aircraft (particularly light twins) are worth (wholesale?)

Total of the core value + time remaining on the engine(s) and prop(s) +
value of avionics and radios or other easily removable equipment.

airframe minimal or none

I think that small hobby aircraft seem to add a (sometimes large) premium
for very shiney or special.

Please comment

Blue skies to all