As Openstars mentions, fitting stuff into a smaller space is much more work and expense than extra material for the body. I've watched engineers layout circuit boards for much, much, much less complex stuff and it's quite a challenge.

Then there's heat dissipation. Having owned numerous phones, including things like the S4 and S4 Mini, the mini would get hot doing certain tasks. Far less surface area means it will heat up and reduce performance. (Granted this was years ago, that hardware and Android version weren't exactly efficient).

I'm sure there's other issues like component selection (and sourcing), how many they expect to sell, etc, etc.

In the end, my money is on projections by marketing/sales/whoever.

One of the questions asked ("Why do I have to give 200€ more to get the s24+ just to get these simple features?") was comparing S24 to S24+. While I have not looked it up, traditionally those versions range from SXY (small) to SXY+ (medium) and SXY Ultra (large), but are otherwise the same phone, so I would be surprised to hear if e.g. the S24 was foldable but the S24+ was not?

As for whether it gets more expensive to make something foldable vs. to make something more compact, I suspect the devil is in the details, so ymmv and you just kinda take each option as it comes. Other factors may help mitigate those costs e.g. a younger company trying to break into the big leagues might try to give phones away for virtually no profits in exchange for their increased market penetration (e.g. OnePlus used to be somewhat this way, now they are in the big leagues, more or less).

But your other point, about more materials: no, I believe that it's more complex than that b/c it's the effort of fitting things into tighter spaces that is more constraining. Imagine packing for a long vacation and/or a job interview at a far-away place and you get the idea - if you can fit everything into one suitcase that's good, but a tiny backpack is much harder to accomplish, and to take nothing and just live with what you can carry on your body alone is REALLY tough! (especially if you want all the normal features like not smelling bad) i.e. the materials costs, while not negligible, have not been the driving/limiting force for many years. At least according to everything that I have read, but I am no phone manufacturer!:-)