Spiderman #1 DEC 81 Value?

I have a Spiderman Vol 1 No 1 dated December 1981. Anyone have an idea
of the value? It is not in a sleeve or cover, but has no torn pages.
Just a little brownish on the edges.
Is there a website I can look at that has comic book valuations? I
have others I would like to price as well.
also email [EMAIL PROTECTED]
Thanks.