A lot of the cost of the ATV comes from the development cost and the low number of missions (for clairty I've amortized that cost through the missions planned up to 2014 in the above). Even so it costs about half a billion dollars just to build and launch each one. To be fair, the ATV is able to transfer propellant to the station which is something that only it and the Progress vehicles can do.

For myself, I'm merely an enthusiastic amateur, I've been studying spaceflight closely since I was a child and I've come to pick up a few tidbits of trivia along the way.