Ars Technica reports on a company called Cycle Computing that has built a 30k-core cluster on Amazon EC2 cloud for a client. It costs $1279 per hour to run, including fees to Amazon and Cycle Computing.
The cluster ran for about 7 hours, doing a computation for a pharmaceutical company. It had 3809 compute instances, each with 7GB RAM making 30472 cores, 26.7 TB RAM, and 2PB of disk space.
In the past, Amazon has built a virtual cluster that got onto the TOP500 list.