This is a helpful analysis and very interesting point about AWS not always applying price reductions to reserved instances. What we do know is your upfront payment is always spent based on the current prices and to maximize the savings from RIs you should buy them immediately after a price drop. Because of this your 3 year projection for comparing AWS to Google should definitely model some effect of future price drops I think you would find it has a very favorable impact for Google pricing. Also on a 3 year time scale I think most finance people would also want to include the future value of money - that big upfront payment is something that businesses almost always want to avoid if they can. A tip: if AWS are calling you about buying RIs it is may be an indication that prices are about to drop again, this has happened to me twice now. A never get calls just after a price drop. Coincidence?