"Original" means many things. To a hard-core collector, that means the rifle has all the same parts and finish it had when it left the Eddystone factory in 1918.
To many, it means it's not been touched by "Bubba", the infamous sporterizer of military guns. Could be rebuilt and arsenal-refinished, but it's not been structurally altered.
Guns that fit the first definition of "original" are few and far between, and thus command higher prices. Guns in the second group are much more common and don't bring as much money.
A completely original Eddystone, with proper barrel (and date for the serial number), blued metal, good finish, Eddystone stock in good condition, that would probably be a $600-$800 rifle (or more, depending).
A WWII rebuild with a JA or other brand barrel, green or black parkerizing on the metal, and a military stock in decent shape is a $500 gun.
Add a bad bore, rust, pitting, or any number of structural blemishes and you've got a rapidly declining price.
I bought a Winchester M1917 at Cabela's for $350. It is in a refinished (but unaltered) stock that can be resurrected for the cost of some linseed oil, has mixed parts, and dark park on it. Also had a pumpkin-orange bore when I bought. A wire brush and some Hoppes made it shine like new. Shoots like a champ, too.