Eurail’s website states that, seat reservations run between €10-€15 on average.
There's a saying about how many functional mammaries the average human has.
Seat reservations for us, I'll reiterate, have mostly cost us between 3 and 6 euros apiece. That includes high speed trains in Poland, Czechia, Austria, Sweden, Norway and Italy. We paid no reservation fees at all in Belgium, Netherlands, Denmark or Germany. I understand seat reservations are more costly in France and Spain, but then again their trains are a lot faster.
As to the OP's recent query, the Eurail pass can be used on local trains as well as high-speed regional trains. The passes have been especially useful in Belgium and the Netherlands in negotiating the many scenic towns from our headquarters in Ghent and Utrecht. Shorter day trips to secondary attractions are made so much easier with the pass.