I'm just playing around with Amazon EC2 instances. No matter, what they cost cobblestones/Euros, just for the fun.

Intro: Amazon spot-instances are virtual machines you can hire per hour for a price you set. It's like an auction: If you set the price to 0.03 Dollars/hour for the dual-core c1.medium machine, you will be able to use this machine as long as the current spot-sprice is equal or below 0.03 Dollars. Once the spot-price exceeds 3 Cents, your machine will be stoped or killed, depending on what you've got configured.

Here some random note.

1) Simple spot-instances, like mentioned at http://www.boinc-wiki.info/Installing_The_BOI...on_Amazon_EC2_%28Linux%29 are crap. This Wiki mentions AMI-images at instance-space, which is cheap, but has the consequence that running and pending workunits will be lost when the machine shuts down. Crap, in my opinion - when you take workunits, which you can't possibly crunch, no-go. OK if it's just you, but if there are to many AWS-users - no-go.
2) There are pitfalls with EC2. First, stay "persistent", so that your spot-instance won't vanish. Second, contrary to the Wiki, don't go for instance-space, but for EBS. This will make sure that the machine - once it got suspended and resurected again (with a new hostname, new ip-address, but with the same hostid) continues crunching for the same workunits.
3) Don't forget to set your EC2-EBS instance to "Stop", and not "Terminate". When you "terminate", the instance will go poooof once the current price goes above your set price - and you won't be able to attach the virtual harddrive again.
4) Add more wonky stuff here.

Conclusion: Just checking out EC2 for mass-deployment. Spot-instances are cheap at the moment, for us-east-1d in the mean of 0.25 US Cents per machine hour for a dual-core c1.medium instance. (E5410 @ 2.33GHz on one of my instances atm)
Didn't calculate cobblestone/dollars yet. Need more data.

Cheers, Alex.

edited to appropriate forum content - ErikaT