I needed to start a new EC2 instance today and decided to try out the new spot instances, where you can reduce your instance cost by bidding on the maximum per-hour price you're prepared to pay. Since today's spot price was only 3.5c / hour, compared with 8.5c / hour for an on-demand instance, I was wondering: if I just bid a really high price, say 10c / hour, can I effectively be sure of getting a much cheaper long-running instance than an on-demand instance (since the spot instances are only charged by the current spot price) ?
I suppose it's theoretically possible for the spot price to go over the on-demand price, but as far as I can tell from the data on the AWS site, the spot price has always been well below that.
UPDATE: I've been monitoring the small instance spot price for a few weeks and it's been very consistent, only varying between 2.9c and 3.1c. I wrote up a more detailed blog post on spot instances, including a link to a Nagios plugin I built to monitor the spot price.