He is known for the massive amount of reproductions that were made based on his work. A reproduction will only fetch around $50, but any original signed painting, depending on size and format, could be worth anything from $1,000 right through to $40,000.
You will, of course, need an appraisal to validate the work and confirm it is genuine. www.robertwood.net/signatures.html lists the signatures Word used most; this should help give an indication of legitimacy. This is no substitute for an appraisal though as a good forgery will have a good signature.