A couple counterpoints: for all the time you just spent making a local cluster, you could have spent 1 day negotiating a deal with the AWS TAM and get 50% or more off rack rates. So that $861K is more like half that. And second, that machien would be collocated with infinite storage and other processing systems.

The CPU/GPU part of training is only part of the much larger ecosystem, which is what you're paying to be in when you use AWS.