I am curious what an "expensive" one would actually cost, too... It is a car so already a large purchase. I'd pay a bit more for an accurate thermostat.
It's not just material cost, probably different interface to the sensor so factor in some R&D, approvals, etc. Any time anything, no matter how small or innocuous (bracket, cable, screw, some piece of plastic cover, etc), was changed on a vehicle, it meant a different part number, which meant 6-12 months delay. This is because it has to go through all the testing - usability, fatigue, safety, etc all over again. This is why they pick cheap parts - not because they're cheap, but because they are old and got cheaper over the years, because old = already approved, which makes the lead time a lot less.
One of the problems is the heat from the device itself, as well as limited airflow creating localized hotspots.