Seat 61 has a good list of reservation prices, including sleepers.

https://www.seat61.com/interrail-and-eurail-reservations.htm

Quote:

Rome, Florence, Milan or Venice to Munich, Salzburg or Vienna by Nightjet sleeper train:

Seat €13.50, couchette in 6-berth €32.80, couchette in 4-berth €42.40, bed in 3-berth sleeper €52, bed in 2-berth sleeper €71.30, bed in single-berth sleeper €119.50, berth in 3-berth deluxe sleeper €71.30, berth in 2-berth deluxe sleeper €90.60, berth in single-berth deluxe sleeper €138.80.  Only a 2nd class ticket or pass is needed for all Nightjet accommodation types, as from 2017.  Exact prices may vary slightly.