I got mine from my local gun shop for $300. pics in here under a thread entitled "Linseed Oil for my Mauser Furniture", or something like that. might give you an idea of what kind of value you should be seeking (mine is in excellent shape).
I would be wary (read very wary) of an on-line purchase of such a thing. No personal experience with Mitchell's, but to say they have mixed reviews would be being kind.