well whole bikes do have a smaller markup than small parts.. have you asked at your LBS? $2.50 (your link) seems cheap enough..

my Silca has kept working for 30+ years and still works .. Once I quit pushing the gasket past the smooth band separating
the coarse cap thread and the fine , larger thread for the ring Nut the gasket wear went to near zero.

It seals in contact with that smooth ring.

EVT | Urethane Pump Washer in Urethane elastomer instead of Italian Made ones in natural rubber .