Not a math major, but I do understand that the value of 100B a month/52 isn't of much use. The yearly cost is 100B * 12 months, so 100*12 = 1200/52*3 = 70B
Oh, and the estimate for FY2007 and FY2008 is up to ~120B a month, but those are from sources other than the Library of Congress.
So...