I think the Kahn academy has got good licensing terms, and dont generate lots of rows of data, or dont need to process lots of data. As a fairly static web app host GAE can't be beaten. If you do more with it I think you will face the same problems we had... http://www.moneytoolkit.com/2012/08/the-problem-with-google-...
At Khan Academy, we actually have terabytes of data; some of our tables have close to a billion rows.
Our backend does much more than just serve static files -- you'd think that we don't have to do much but we keep track of video and exercise progress, summarize progress by user and by coach, provide video recommendations, host a Q&A system with notification support, award badges in real-time while avoiding costly datastore queries, handle mailing lists and email subscriptions, and have support for having a public profile to show off your progress, among other things.
We handle hundreds of requests per second and it's really nice to not have to think about scaling very much -- with any other solution, we'd be doing a lot more system administration.
Wow, my bad! Thanks for pointing that out. If you scale my numbers up that would be costing you $60,000 per month. If you are doing recommendations you are probably processing significant amounts of data as well. I guess theres no way we could know the commercial arrangements, but It surely cant be the same as your average small time developer.
I have to admit that I didn't think about this much myself -- most of the videos are on YouTube, and I hadn't thought about how the rest of the site was run. For all I knew, it was RoR or static webpages or something.
although I have not used app engine much, it seems like you would have a lot less flexibility if you use it rather than EC2/Rackspace.
Also, Im not sure I believe statements like "its nice not to have to worry about scaling", although if it is working for KA at hundreds of requests per second, maybe it is an accurate statement
