If any of his work has ever been sold at auction then Sotheby's and Christie's
will have records of the sale as a matter of public record.  If he has been
sold at a smaller house then it is a little harder.  People tend to cringe at
the thought of using the big auction houses as sources for appraisals and
prices, but the publically paid price at an auction is a valid basis for
replacement value appraisals.

EA Marecki
Collections Manager
Kelly Collection of American Illustration