I wrote a stupid little script that was designed to figure out whether it is more cost effective to run my server on spot instances or on reserved instances. It calculates cumulative spot cost over the last 90 days and also charts the spot price, and projects that cost to a full year.
My server must be up 100% and cannot tolerate downtime, so I can compare the projected cost to reserved instance cost. Historically, the spot price has been pretty stable (so stable that one commentator called the market a flop) and almost equal to the reserved instance hourly price, without the overhead of the reserved instance one time fee. However, spot prices have been fluctuating greatly in the last few months (Q3 2011) so it is now more efficient to buy a reserved instance (especially since Heavy Utilization instances became available).
If we project the cost of running a micro spot instance in us-east-1b for an entire year, we would have to pay $621.20/yr for CPU time. Compare the Heavy Utilization reserved micro instances in the same region, which would cost $100/33 + 365*24*$0.005 = $77.13 (3 year term). Yeah. I know which one I’ll be getting.