Quite the collection you've got there!
Approx. how much does one of those things cost to make?
It really depends. In the 40% family picture, there's this one custom one - the kb45p (which is the one on the bottom left). That's my cheapest one at about $100. You can make them cheaper- I just had to get custom plates cut which drove up the price. That purple one that I just made on a whim was about $102. Both of those prices are without the keycaps.
The economy of scale and quality come into play quite a bit. One-offs are inherently more, and if you can get a lot of people to join in, they're inherently less. And mass market is definitely a lot less. But for a keyboard in a reasonable range, of reasonable quality from a storefront, I'd say about $150 or so without the keycaps.
My wife and daughter's keyboards, I started with a standard keyboard I purchased from amazon, and just got the custom keycaps for the boards. So those were a lot less, as they keycaps on those are from non-top line manufacturers.
My daily drivers (the two clueboards) were $180 just for the keycaps, and the boards themselves were $229 for the dent and ding board, and 260 for the non-dent and ding.
Then I had to provide the switches; I tend to get them on massdrop in bulk when they have them, so they were just sitting around. For 120 gateron brown switches (which is what I tend to run) it's about $25 from there.
My most expensive board is probably my JD45- that was a complete custom with custom case and bluetooth. OR my ergodox, which I haven't put together yet: 276.98 for the kit, but not sure if I'm going to switch out the keycaps which would drive it up more.