Pricing for builders plates?

How does one go about finding the fair market value for builders plates? I have tried google but not come up with any answers. The plates in question are early 20's Porter plates.

Roger

This is a unique find! A builder plate off a 1940s Pullman car ex-PRR. This plate is stainless with black enamel detail and lettering. No dents. These plates are off the end doors of Pullman cars located in a long line during the early 70s in Texas City, Texas on the way to Mexico for scrapping. There were several 1948 era blunt end obs-sleeper cars there off the Broadway and other PRR crack passenger trains of the era. A rare item!! Seldom seen. (I have a list of the cars that were at this site for the historian and collector who buys this plate) Sold for $54.00 http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 32729&rd=1[/url]

This rare original stainless steel Builder's Plate was used on a Santa Fe passenger railroad car which was built by The Budd Company,Philadelphia in 1954.It is made out of stainless steel and measures 3 1/4" x 6".This plate was mounted on the vestibule end of a passenger car after it was built.The plate is in very good condition.I also have a 1952 Budd Builders Plate in this auction which. SOLD FOR $15.16
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 23122&rd=1