My guess after building it would be that it would come in around $100 to $150 depending on what you have around for 3D printing and so on. Unfortunately, I recall there being a bit of a jump in component prices circa 2020 that really hasn't resolved itself.
As one of the other commenters mentioned, keyboard switches don't come cheap. Your best bet would be to look at some of the sources for Cherry MX clones, though for my build I was able to find a US-based source for Cherry MX switches relatively cheaply. It also helps that you don't need nearly as many as for a full, traditional keyboard.
Another cost to consider is the cost of the PCBs. All of our boards (including the prototypes that didn't go anywhere) were done by Aisler and were a bit more expensive than Chinese board houses, but they always came through and I was too lazy to shop around. Per board the cost isn't bad, but consider that you're buying boards in groups of three, and then you have at least two PCBs (the main board and the keyboard).
It's also going to depend on where you live and what kind of import duties you might get hit with.
As one of the other commenters mentioned, keyboard switches don't come cheap. Your best bet would be to look at some of the sources for Cherry MX clones, though for my build I was able to find a US-based source for Cherry MX switches relatively cheaply. It also helps that you don't need nearly as many as for a full, traditional keyboard.
Another cost to consider is the cost of the PCBs. All of our boards (including the prototypes that didn't go anywhere) were done by Aisler and were a bit more expensive than Chinese board houses, but they always came through and I was too lazy to shop around. Per board the cost isn't bad, but consider that you're buying boards in groups of three, and then you have at least two PCBs (the main board and the keyboard).
It's also going to depend on where you live and what kind of import duties you might get hit with.