So that is about 55 million dollars a year to completely secure the network and process all transactions. I am not entirely sure how it will scale, but I predict that it will scale logarithmically with the number of transactions, especially considering that mining equipment will become more efficient.
Meanwhile the United States will spend 797 million dollars [1] this year in simply printing and transporting physical currency, not including all the other obvious costs in maintaining a modern currency.
[1] http://www.federalreserve.gov/foia/2013newcurrency.htm