Hi, ag:
I own a fullstock Winchester P14 which is good-looking and a good shooter. As I am sure you know, P14s were all made in the U.S. by either Remington, a Remington-owned factory called Eddystone, and Winchester. The time frame involved is very short: 1916-1917. Then the pattern was used for the U.S. Model of 1917 in 30-06. About 4 million of the .303 version were made. Winchesters are the "rarest", but not by much. $400 is a fair price. Bayos go for over $100, maybe $150. Ideally, of course, you would match a Win bayo to a Win rifle, but I don't think they matched bayo serials to receiver serials.
The most interesting part of P14 collecting might relate to the volley sights. All (most?) P14s were made with volley sights: a dial and pointer mechanism towards the left front of the stock and a peephole on an arm swinging up from the left side of the receiver. This was used for the interesting, but ultimately unworkable, idea of raining bullets down upon the enemy from 2000 yards.
After WWI ended, the P14s in Britain were put into storage but first (or at some point) they were sent to the arsenal for modification: removal of the volley sights. So, you will rarely see a P14 with sights intact, or sights available as parts to the collector market. An intact P14 (somehow kept by an original issuee) would be worth much more than the regular kind.
Then as WWII opened up, the P14s were brought out of storage for Home Guard use. There should be some markings of interest to collectors out of this phase. (The Home Guard also used Model of 1917s, necessitating a paint flash to distinguish the calibres. A rifle with stripes in red/white/red on the front stock and forestock is chambered in 30-06.)
A really serious collector would be looking for experimental versions of the P14 in .276 calibre - this was how the rifle was chambered by the Enfield people just prior to WWI. When the war broke out, they sent the designs to the U.S. and asked for .303 chambering to maintain ammunition standard with the Lee Enfields.
Anyways, you may know all of the above, but I enjoyed blabbing on about my favourite military rifle.
P.S. a snob would always call it a "Patt.14", because that is what is marked on the buttstock cartouche.