They're expensive because they're no longer made, and Ansel Adams was famous for promoting the use of them for the Zone System. A lot of his followers who themselves because famous also used them and promoted them, because Adams did.
They're not the best spotmeters. They're quite primitive, only reading to 1/3 stop accuracy. Minolta's Spotmeter F is a lot cheaper, more accurate, and can read flash if needed. It is small, like the Pentax. An even better choice is one of the Sekonic models that do incident or 1 degree spot. They're modern highly accurate digital meters, still manufactured, and are not very large...plus you get incident metering as an option for when you shoot slides or digital (which do better with incident. Spotmetering is best for negative film).
There is absolutely NOTHING about the Pentax digital spotmeter that makes it worth the idiotic amounts ebay crooks are demanding. Nothing. Save your money and get a modern meter. I love my Minolta Flash Meter VI and wouldn't trade it for 10 Pentax meters. It sells for less than a Pentax used and does ten times more.