Your other potential issue is in assuming the difficulty rate will be constant for a whole year--in fact, as long as mining at any scale remains profitable, it is likely to increase continually.
Got it - however, as I understand it, as the difficulty increases and profitability decreases, fewer people will find mining worthwhile, eventually leading to a reduction in difficulty if the number of blocks being solved drops below the required rate to keep the market going. There must be some steady state and I wonder how close we are to it. I have heard others argue that it's unlikely we'll see the same pace of difficulty growth in the near future as has been the case for the past several months.
Also figure in the people that don't care or don't realize they're losing a little and a few other cases.
