There are six catalogues in the Vince Clarke collection, the first five for the Science Fiction Service, the sixth for Milcross. By way of looking at their pricing policy, here's a table comparing back-issue prices for one magazine:
From the table above it's clear that when they started out they would ask more for a magazine the older it was. The drop in price between December 1938 and April 1939 suggests that perhaps the magazines weren't selling and so a reduction was called for, although by October of that year prices have recovered. By 1954 it seems that rarity rather than simply the age of a magazine is determining prices. New issues are going for 3/- but most of those between 1935 and 1952 are selling for 1/6, or half the new price. Clearly there was no shortage of these issues and they could probably be picked up cheaply and easily on used magazine stalls. The exception to this rule, for fairly obvious reasons, are the wartime issues. The June 1940 catalogue lists no issues for that year, while the Milcross catalogue lists only a single wartime issue - June 1943 - priced at 4/-.