The original Thun shape is rather great, but you can get similar, or identical cups for much less. Of all the money that an espresso nut will blow on his hobby, 100 bucks for 4 espresso cups isn't that mad though tbh
But yes you're only paying for the design. Then again, people collect stamps/coins that collect dust in drawers and cabinets. If I'm going to be collecting inanimate objects, I'd much rather have them functional, and pretty to look at, but that's just me.
I don't exactly collect them, but I have the pen drawing set, the aliens and body parts.
The cups themselves are porcelain but since they've been made by different manufacturers the exact weight and thickness of the cups has varied through the years.