Most download sites and physical retailers are currently charging £5.99 or over for either Deus Ex or its sequel Invisible War. Thing is, though, you can get both games for a fiver thanks to Amazon Jersey and Play.com.
Look: if you haven't already played Deus Ex, you should correct your mistake immediately. It's generally considered to be one of the best... if not the best... games of all time. And you might as well get its inferior sequel to boot.