It can be cheaper for very limited situations, sure.

I have never seen a case where AWS makes financial sense for the base load, or for anyone without that severely bursty traffic, though. For batch jobs or handling peaks of less than 6-8 hours, sure. But fewer people have peaks that are economical to handle that way than they think. And for those who do - e.g. retailers that needs to handle the christmas rush, depending on a system where servers can in theory become unavailable at any time is a high risk game.

Most of the time you can get to 30% of AWS with rented managed servers. Racking your own kit can get substantially below that.

Going hybrid is interesting, though, as it actually makes the case for actually using AWS much even weaker for most people:

Whereas previously you'd need your managed servers to be able to handle sufficient peaks to give you time to spin up more servers, once you have support for spinning up cloud instances for peaks, you can often aim for near 100% utilisation during normal days peaks, which magnifies the price difference substantially.

Even for most e-commerce sites, unless your site has a very localised audience, peaks are rarely high enough or sharp enough to make AWS economical if you have a system that's prepped for a hybrid situation - you can let things get very close to the wire, and start spinning up VMs, but you very rarely will find situations where it's worth doing for more than a few days before it'll be apparent it'll be cheaper to spin up a managed machine billed monthly, and/or putting in an order for more kit to put in your own rack.

> Not having lots of idle capacity that still has to be managed and paid for,

You are still paying for it with AWS with every single hour of VM's you pay for - it's one of the reasons AWS is so expensive: You have to fund all the spare capacity they need to have on standby to prevent running out of hardware during peaks.

> not having to beef up the Ops/IT staff,

This attitude scares me. You have to be prepared to deal with outages of pretty much every kind with AWS too. If you run managed servers, there's pretty much no difference, other than that you have to account for whatever the hosts turnaround time in bringing up new servers is (which one of those places where a hybrid setup might help you - especially if you're using a host like e.g. Softlayer or Iweb that offers the full range from managed servers to cloud servers in the same data centres).

The one thing you don't have to deal with is if you use a colo and it saves you people to cable up and rack servers and install the OS. But you can hire people to do that too.

> not having to own your own/manage/maintain hardware, etc.

... but you don't need to do this to beat AWS hands down, either: If you rent managed servers on a month by month basis, with no contracted minimum term, you can still easily come out vastly cheaper for the base load and most peaks.

> My advice to others is to be wary about generalized figures like "20-30% of cloud cost" from anyone with a vested interest in getting you to switch. Generalizations are dangerous when every usage case is different.

Sure. It's sound advice to do your own maths. But every time I have, for a variety of different sites, AWS has come out ludicrously expensive for anything but batch jobs and very short, sharp peaks.