Are container homes actually cheaper to build?

One thing that's common here is to make a barn with a road base foundation, and stick a metal roof on it.  That makes rain catching easier and gives extra storage in the triangle of the roof.

I'd like a couple containers for sheds, but probably not residences.

Yes, so much depends on what your goal is:

1. If you think you might need to move your house, containers are designed for that. Yes, it takes big machinery to lift it onto a flat bed, but like a turtle, you get to take your home with you. I know people in situations where that would be a major asset, regardless of whether the build turned out "cheaper" or "not quite".

2. Cost is simply not always a person's only concern. I'm sensitive to many modern things that off-gas (formaldehyde in particular). Building your own home in a container allows you to control for that, but also tends to increase the cost. If you balance the cost with health care costs, in the long term it most definitely could be a win.

3. I will also point out, that metal walls on the outside, so long as all the trim prevents ingress of bugs and mice, in areas where termites, carpenter ants, and mice that can squish through amazingly small holes are endemic, they will not be a concern! Again, there are people out there more concerned with long term value and mental security, than the dollars up front.

I suspect that the variation in cost will be as much down to the requirements of the individual than the building format itself. What extra costs would having a container home involve? I'm thinking that ventilation is probably the only one that wouldn't be required to the same extent in any other build method.

I do like the idea of a modular design. Then you could add on another container in the future to double the space - or even triple it with a dog trot type design with a roofed space in between two containers.

I think it's easy to look at a rigid metal box and say "all the hard stuff is already done!" but I think the hard stuff when it comes to construction is mostly in the little details where one component or piece of material meets another, especially if they are not combinations you have optimized your skills and tools around.

Shipping containers work really well when used as big storage boxes with doors at one end. As you try to make them incrementally more like houses the challenges pile up, and in my experience working in them, few things are as easily installed in a shipping container as they would be in conventional construction. So I'd say plan for everything to be harder and cost more.

I watched a video series where a guy showed how he converted a shipping container into a small office. He cut penetrations for utilities and a single window, sprayfoamed for insulation, and finished the interior in plywood with surface-run electrical outlets & switches using metal boxes and EMT conduit.

It looked like he struggled to cut the penetrations and flash the window. This was a very capable and skillful guy in the video using probably excellent tools (Youtube channel is called Essential Craftsman) and even he ran into problems and had to redo stuff. He said it was due to being old but to me it read as a warning that a less experienced person of any age would have had an even harder time than he did.

Containers can save money on structure

The word "can" is doing a lot of work there. I could easily envision scenarios where they don't save you any money even just on the structure. Conventional stick-framing can be done extremely inexpensively and quickly, for a structure that size.

Where they shine:

Faster build time

It's less time before you have a roof over your head I suppose, but I think preparation and experience are going to be the limiting factors in terms of speed to completion.

Don't get me wrong, I think shipping container construction is cool, and I'm happy people are trying it, but it looks to me like it is deceptively challenging.