A video has been posted on SecurityTube that shows in detail how to create a GPU-node in Amazon’s EC2 cloud and get Pyrit installed and running there. The measured performance of the node (defined by it’s type == $$$ you are willing to pay per hour of runtime) is around 50.000 PMKs per second. That’s not very high but not bad either. The point is that you can setup and run (== pay) and shutdown (== not pay) the node anytime you like – there is no fixed cost for hardware. If you want more performance per second, you pay more per second.
The video is about one hour long so there should be only little room for uncertainties.