We in the library are not in a position to determine the value of a painting/sculpture/drawing, etc., but have access to various sales indexes available on the list of art databases.
Alternatively, try the International Guide of Art Experts, Specialists and Catalogues Raisonnés, 2 vols., Ref. N50.K53 1993, or consult Art Market Research: A Guide to Methods and Sources, 2nd ed. by Tom McNulty, Ref N5200 .M39 2014. Another suggestion would be to contact an appraiser via an organization such as those listed here.
