cudaHashCat64 on AWS EC2
What no one seems to have mentioned here is that they can be run in parallel. Sure for the price of ~10 days of processing you could buy a card that is good for years, but if you need it urgently, amazon may have an advantage.

If you break the task down, you can run that entire 10 days worth of processing on a couple of hundred instances and have it finished in an hour. You can't achieve the same kind of flexibility with physical machine.

Long term physical hardware is the better investment, but if you have extremely bursty requirements, letting someone else deal with the machines might be the better option.