[caudicus]

Reserved instances (assuming you're going to use it for the full year, which of course is the big caveat) also bring the cost down to $0.007 an hour with an up-front investment of $54, so:

(($0.007 * 24 * 30)*12 + 54)/12 = $9.54 a month assuming you use it for the full year.

Edit: Also, as mentioned by _delirium, Linux instances are also $0.02 an hour. I did the Linux calculation but the same one with Windows goes at $0.012 per hour.