In interesting cases, cost is _very_ significant. And it's not a 2x factor, IIRC it can easily be 10x.
I am assuming nothing. I personally had this very same debate with a program manager of a company whose bread and butter was doing a lot of computation. At the eyes of upper level management, arguing about spending 20€ or 100€ on a cloud service provider is a kin of debating which brand of detergent should the company buy. It's irrelevant.
> But in that case, a company could just rent a server at some ISP and be done with it.
That's where you get it all wrong. Hiring bare metal services does nothing to your ability to scale, either to meet demand or to develop/test/try out new services, nor does it help you use higher-level managed services. Everything you have to do or manage by yourself is a productivity hit, and that productivity hit is measured as a percentage of your entire payroll, which eclipses how much you pay to your cloud service provider.
... although if it's outbound bandwidth - if you can take care of your compute, maybe it's possible to purchase outgoing bandwidth at an hourly resolution, rather than "whatever your wires can send us", and have your flexibility.