Putting A Value On Precancels - How?

I used to collect precancels, particularly on Back of the Book stamps.
How do I value them?
Here are some examples:

Well, you can go to ebay and check what some of them are selling for or better yet got to the precancel society's website and order some of their literature. Www.precancels.com .


Peter

ebay is close to worthless since Back of the Book precancels of what I've got rarely if ever show up.

And I have 2 older publications of the precancel society:
-- The Precancel Stamp Society's Town and Type Catalog of the United States and Territories, 7th edition, 2007
-- Precancel Stamp Society Catalog of United States Bureau Precancels, 5th edition, 2005
Except for ID'ing precancel types I don't think they are much good for pricing.

What I've been thinking is to price the stamp from Scott and add a little bit for the precancel.
For selling purposes auction them starting about 20% of that calculated value.

the StLouis is listed in the new PSS Classic Catalog at 15$ each .... the others are very common denominations so small premiums