How Do You Value A Plate Number Single?

Here's the problem. The stamp pictured is J57, with a Saint Louis Missouri precancel (type L-6 E). It is a rather scarce plate number single. The stamp itself is valued at $55.00 in used condition (since it is precancelled), while the plate block is valued at $2750 in unused condition. There is no valuation for a used plate block.

With the plate number single, what's a reasonable valuation to place on it for insurance purposes? No, it's not for sale!

You might visit http://apnss.org ... the American Plate Number Single Society. There is a cataloue for these ... Hebert's Catalog. I don't know any details, but perhaps you can contact someone on the web site. Or perhaps there are members of that group that frequent this forum.
Added: Just looked on ebay & found a couple of 1992 Hebert's catalogues available. From what I gather, the 2011 edition is the most recent. It is availabe at apnss.org.

As a quick and dirty approach: the 1999 Scott catalog listed a normal used single of J57 at $17.50. The 2000 edition of "Hebert's Standard Plate Number Single Catalog" listed a used plate number single at $25. (The only edition I have) So about a 40% premium over Scott for the plate number. And while hazardous to do such, scaling that against the current $55 Scott value would give about a $75 catalog value today for your stamp with a plate number, with no adjustments for centering, soundness, etc.

John, That sounds reasonable. I appreciate the analysis of the two volumes. Looks like there's another book to add to the library!

Stampman, you ought to have included the word "precancel" in the title. We have a couple of precancel experts on this forum and they would be happy to give you an idea.

Peter