Cheapest I have seen these.
Play.com = £39.99http://www.play.com/Electronics/Electronics/4-/8852229/Sony-X-Bass-MDR-XB500-Headphones/Product.html?
Amazon = £30.97 (but there wasw a 1-2 month wait for them)http://www.amazon.co.uk/gp/product/B001UE6PDG
These Sony Bass MDRXB500 headphones come in black complete with thick, pressure-relieving king-size urethane earpad cushions for extreme unbeatable comfort all day long or night long. Experience deep bass with the XL 40mm XB driver unit, for a rich bass sound that brings you closer to the club music.A carry case is included with these headphones that have a flat cord with a length of 1.2 metres for ultimate tangle-free flexibility and freedom while listening.The MDRXB 500 headphones have a tightly-sealed acoustic design for superb sound isolation, with the neodymium magnet delivering clear mid-range sounds.