Thank you for sharing these Whizard. Although not common, these do appear on eBay with some regularity and sell for modest prices. Postal censorship markings first came into widespread use during the Boer War and a variety of censorship stamps and seals were used on both sides of the conflict. Of particular interest to collectors are those to/from the British PoW camp for Boer prisoners on St. Helena.
Going by the large number of such covers regularly listed on Ebay they do not appear to be very scarce. Values seem to have a wide range depending on markings and whether they were used from POW camps or unusual locations or have rare postage stamps, etc. Ignore the starting prices and asking prices and filter for ones that have actually sold to see what they go for. Some of the more common ones seem to be in the $3 to $10 range, and some of the rare ones go up from there and a few of the rarest are into the hundreds.