That Interrail can't tell you the price. They can't sell the reservations for Spain (or Portugal) anyway. By the way, there are also a few reservation-free trains Segovia - Madrid, from the “Segovia” station in the city. Segovia Guiomar is 5 km from the city and you need a bus to get there.

Be warned though that the rail planner app is not reliable enough to plan: it is only updated about once per month. www.raileurope.com is more up to date for Spain.