Fantastically long-running and collectible, early issues anyway.
They show up on eBay UK quite regularly in average condition and sell for between £1.50 and a fiver for the most part it seems. The difficulty is that the numbering on the cover stops quite early on so numbers to years is tricky to work out.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.