I thought the same until I tried a couple, and then tried going back to a membrane keyboard at the office. In addition to better wrist comfort, I also find that I have fewer typos on the mechanical keyboard - my intuitive guess is that it's due to greater travel and resistance required to actuate a key. Everyone has their own preferences but, for something I use 8 hours a day as part of my profession, $100 for a decent keyboard seems entirely reasonable. Plenty of contractors have fancy trucks/tool boxes/tools that cost way more.

Also, it's not comparable to a stool for a guitarist - that would be my desk chair, which cost more than the keyboard.