First it would be good to confirm if it's German Westfalia or an American campmobile.
Campmobile made two models a basic and a deluxe, which could be fitted to panels, kombis and microbuses, and you could get them with various accessories. The easy way to tell the two models apart is that the deluxe came with a water pump and a cool box unit a chemical toilet and a two burner hob but these could have been lost a long time ago. It also had two fold down shelves one on the rear side load door and the other on the side of the unit just inside the front side door.
Cost all depends on condition and as to how original and complete it still is but as previously mentioned The samba.com would be an excellent starting point. A copy of the book VW camper- The inside story (ISBN 1-86126-763-0) may help to decide what you have.
A little knowledge is a good thing, except when in the hands of the stupid !