I'd noticed it too - the prices for bottled water in particular are a complete rip-off.
You don't need to buy the headset - if you plug in any headset from a personal stereo it will work, but only in one ear. That's adequate for the inflight movie.
But if you really need stereo, take 2 "earbud" style headsets.
You can plug one into each socket and still get stereo, using one bud from each pair. Earbud headsets are available for under a pound if you shop around.
Also, you can use the headsets bought on the outbound journey again and again.
Or just keep the adapter that comes with the headset in your hand luggage and use it with your normal headphones.
Supplement from 06/24/2008 10:37am:
The in-flight meals are a complete ripoff too.
Now that many operators charge for the meal as an extra, I always avoid them. £15 for a cruddy meal? I don't think so.
I can manage 3 hours without food thankyou very much.