That is, "Most Nightjet trains use Comfortline sleeping-cars built by Siemens in 2003-2005 for German Railways' City Night Line sleeper trains." and "Below: Comfortline sleeper layout. All compartments can be sold as a single, double or triple."
I picked 14 April 2021, leaving Wien Hvf (U) at 20:13 arriving Berlin at 08:55. "Sparschiene Nachtverkehr Nightjet + Anschlussticket Nightjet." "Sleeper Bed", "Compartment with 1 bed (Single)".
€ 216,00.
Bear in mind the comment at https://www.nightjet.com/en/ : "Due to the travel restrictions caused by the corona pandemic, we are reducing the Nightjet traffic to probably 8 February 2021 ... As a precautionary measure, no bookings are currently possible for the affected trains from January 10th to March 24th, 2021", confirming johannes1234321's comment "During Cornona this probably is different".
That is, "Most Nightjet trains use Comfortline sleeping-cars built by Siemens in 2003-2005 for German Railways' City Night Line sleeper trains." and "Below: Comfortline sleeper layout. All compartments can be sold as a single, double or triple."