"the publishing pattern [of the first six books] was basically the same as for I, the Jury: small first printings (about 7,500 each) and a few reprints followed by multiple printings in paperback and huge sales. Signet issued a 2.5 million copy reprint of The Big Kill [the fifth book], at the time the largest printing in paperback history."
The price for the White Circle edition in the checklist is $100 - $250. This is still a good price range. The book is not rare. There are 13 copies for sale on ABE, seven of the first printing ranging in price from $15 to two copies at $300. The condition of the $300 copies do not support that price. The $15 is very fair for a poor but complete copy.
Wm. Collins Sons & Co. Canada Ltd published its White Circle imprint from 1942 until 1952. In late 1948 I, the Jury was published as CD384. A second printing followed in 1949. Note that the cover colours are slightly different on the reprint. The reprint is clearly marked as such on the copyright page so one doesn't need to compare the cover colours to establish the printing. There is a dealer on line who notes a third printing which doesn't exist. Unfortunately this has found its way into Holroyd's Paperback Prices and Checklist. Even the Firsts article talks about "later printings."
White Circle CD384
White Circle CD384 second printing