Check Earl's Repair Service. Probably about the most knowledgable Walther person in the USA. His web site has pictures ( not very good ones, though ) and parts diagrams of many of the Walter rifles.
http://www.carlwalther.com/
Also check the International Walther site. There is a guy on there named Dieter Marschall (sp???) who knows EVERYTHING about Walthers. He wrote the definitive book.
http://www.carl-walther.de/englisch/forum.html
These are really neat rifles, but parts are, of course, nearly impossible to obtain. The Walther plants and all the drawings and such were destroyed in the war.
Establishing a value is apparently very difficult, since there are so many CRAZY collectors that drive proces up. Dieter will not even speculate on value, seeing as how he lives in Germany. A lot of it is how much do you want it taking into account the condition, parts availability, etc.
I have seen some sell for $350 or so recently, but I bought one on here from another RFC supporter for less than $200. It had a broken piece ( the " ball " on the end of the bolt handle ), but I looked at it as a parts source for my other Model II.
I have seen others like the meister series of the modelV which is a single-shot for around $600.