[Redmagster]

The reason for the differences in value are generally due to scarcity and desirability..

The term proof came into play late 18th century when a small number of prints would be produced for collectors, usually before any inscription was added to the final print. Since prints weren't usually numbered or signed a proof only had real value if it differed n some way from the final print.

Now that editions are often numbered and signed a proof either differs materially from the finished print in that it somehow shows the work in progress (the MacFarlane Kennedy figure being a prime example) or is a limited run given to the artist themselves and not designed to be sold but rather gifted to friend or those involved in producing or distributing the artists work
The Kennedy piece is unusual in that the proofs are numbered sequentially to the overall run - ie they are 1 to 50 then the run itself is numbered to 950 so the total run includes the artist proof.

Art historians and collectors view working proofs as especially desirable because of rarity and insight into the work in progress and because they may have belonged to the artist.

Collectors also usually prefer final artist's proofs even when they are identical to the main edition since they may have been given to a friend by the artist.

An artist's proof has special value because of its extra rarity and its possible differences from the "standard" print, factors that are often reflected in its price.

The number 1 of any limited run is always valued higher than any other number in the run - purely down to the competitive nature of the human heart...