Sibton's link only goes to a page of search results, which actually fails to find the product sought.
Given that the manufacturer's recommended retail price of Prolotex socks is US$48.75 per pair, they're bound to be expensive wherever you buy them!
(That's about £31 at current exchange rates but, allowing for import costs , you'd be lucky to find a UK retailer selling them for less than £40 per pair).