>What if you don't have sustained usage?

Well, that's where cloud servers are great.

You'll need to break out Excel and calculate whether it's worth it with regards to usage.

Isn't it what beowulf clusters are for?

It's never cheaper in the cloud.

No. Scientific workloads rarely scale well when having to do a lot of communication over a commodity network. You're also assuming people would rather have a bunch of machines lying around that they had to pay upfront for than just paying for an occasional single instance? You're oversimplifying things if you literally think it never makes financial sense to use the cloud. It's the same as saying it's never cheaper to have health insurance. Objectively, that has to be true on average, yeah, but then why do so many people buy health insurance? You're managing complex risk at the expense of overhead. Even when it sucks, it's not feasible for everyone to keep enough cash laying around for when they get hit by a bus.