I've been spoiled, so I'm not sure if its a rental fee or free in coach for Transatlantic flights.
photog: i'm not sure if RadioShack sells them; i can't find it listed on their website. I belive you can pick them up at most gadget shops in the airports. I had actually picked up mine from an audiobookstore for $4.
Update: Buy.com has them for sale for $5.99 - not a bad price. Search for "airline adapter".
-JC