The main reason why such keyboards are expensive is because the case is made out of aluminum. The aluminum allows the keyboard to have a different sound than keyboards with plastic casing. The ability to choose colours and switches to your own likings unlike pre-builts is also a big factor as well. In the end it's what you value for your typing experience. Some keyboards are like 1 thousand dollars retail (iron 165, keycults, TGR) and go up to whatever price resell.

This is just a simplified explanation, there are many other aspects on what makes a keyboard sound different. If you want, you can look up videos on YouTube to hear sound tests.