The incremental cost of producing single-injection-molded keycaps is probably only a few bucks per keyboard, but the tooling and overhead is pretty steep, so anything done in small batches ends up being very expensive. Double injection molding or dye sublimation probably costs a bit more, but again most of the cost is fixed overhead, not per-set.
If you have 100,000 identical sets of keycaps you want to produce, you can get someone in China to make them for very cheap. If you only want 10 sets, you’re going to have to pay a lot for it.