I wonder what would happen if you offloaded the rendering of a movie like Monsters University to something like Google Compute Engine. Would it cost an arm and a leg? Or would it solve a lot of scaling/cost/time challenges?
I think Pixar will do low-resolution runs all through the day (so people can get immediate feedback on what they are doing), and a high resolution run overnight (so they can see the current product). If you're at close to 100% utilisation, and have a large cluster, renting hardware is very cost inefficient.
The cost would be completely prohibitive. Pixar is at the point where they have to micromanage the electricity cost of their rendering clusters. (At one point they did a multimillion dollar hardware upgrade on the grounds that they would actually save money thanks to new hardware being more power efficient.)
It would cost an arm and a leg... it would be interesting to run the calculation though. There are a lot of bottlenecks in running a render farm like that, but one of them is just moving the data around. A typical scene uses upwards of 1TB of data (textures, etc) and every scene has a different set of data. So there is a lot of local caching on a fast SAN. Since the cost of hardware is amortized over many simultaneous productions and the farm runs at pretty close to 100% 24/7, I think the hardware investment is paid for quickly.
I once tried offloading rendering (Blender to be exact) to Amazon EC2. It turned out to be extremely costly. Rendering a 3-minute video at 1080 would have cost me $60. Yeah, peanuts for Pixar, but remember this is 3 minutes at 1080, not the ungodly amount of pixels they have to render. Also it was a pretty simple scene; a more complex one would have cost more.
This came up in a discussion I had with a tech at Disney Animation. The biggest issue is actually getting the assets from the workstation over to the rendering farm. Since the assets for a single scene can be many gigabytes using someone else's far away render farm would be prohibitively slow.
I actually wonder if Google, Amazon etc have enough idling servers to suddenly handle rendering a 90 minute movie (at approx. 11.5 hours / frame, if not more with these new lighting technologies).