My back of the envelope estimate. These are based on the figures from last year and the fact that they currently have 240 EC2 instances, some are large (guessed 70), more are x-large (guessed 170).

8760 is the number of hours in a year.

(8760 * $0.24 * 170) + (8760 * $0.12 * 70) = $430,992/yr in hourly fees

($1,820 * 170) + ($910 * 70) = $373,100/yr in reservation fees

373,100 + 430,992 = 804,092 / 12 months = $67,007.67/mo

Reference for last years calculations: http://www.reddit.com/r/blog/comments/ctz7c/your_gold_dollar...