I was floating in the audio department whilst a lady was putting yellow stickers on the headphones and was very surprised at seeing a pair of Black PX 100 marked at £10.00. The white ones are still 40 odd quid.
Just checked the Tesco online price and they are still £40.84. The cheapest i could quickly find was amazon at £28.99. I don't know if its store specific, but worth a try. BTW I took the last pair in the B'pool store sorry.
Here's the description from the website:
The lightweight black Sennheiser Px100 overhead headphones deliver rich sound to your ears. The compact, portable design is ideal for when you are on the go and they will fold into a small size. For extra convenience, these headphones come complete with a carry case. This product is optimised for use with iPods, iPhones, MP3 players and CD players.