I don't think a good kettle has to cost $90 - There's one one amazon with 80% 5 star reviews for $20. http://www.amazon.com/Courant-Cordless-Electric-Coffee-Maker... . Still there are lots of wealthy people with $100 + to spend so manufacturers will naturally make stuff for those price points if customers will buy it.
I had an expensive kettle which was a gift. It lasted ten years but then started to leak and finally started tripping the breaker. I replaced it with a $20 plastic kettle which does everything the other one did, except boil faster and quieter. Just doesn't look as cool though.
In reality, expensive kettles are like expensive clothes. They are probably made out of better materials, but the rest is just markup to cover the lower volumes and higher marketing costs. And because people will pay it for a brand name and sleek design.