You can find it for £10 (€12) if you order it from the UK. Amazon.co.uk has it for that price right now and other retailers like ShopTo and Zavvi also have it for £13/15.
I might do this later on. First going to check out the payment options, have no credit card and don't ever plan on getting one. I was going to do this for Hyperdimension Neptunia in a while as well since I can't find it here, so might as well get a couple of good deals in. And didn't want to ship from the US, but the costs should be less when it ships from the UK seeing as you can get here from the UK in just a few hours.
Today we have 3,000 licensed KISS products, everything from condoms to caskets. We have you coming and going.-Gene Simmons
It's probably gone cheap now because they announced the DLC so early people didn't trade in straight after beating it, now most fans of Deus Ex who intend to have beaten the Missing Link DLC a lot of people will be getting rid of copies, it's not like it has that much replay value considering it's not a game of 'choose your own path' so much as 'choose a path and then go back and take the other paths for the extra exp' and the story differences and endings don't significantly alter the experience.