There are two 3* hotels close to where I live near Adenauerplatz/Kurfürstendamm. One is called Panoramahotel/Adenauerplatz
and charges about 80 Euro for a double incl. breakfast. It is on top of the
U-Bahn Adenauerplatz which takes you to DOB either to Bismarckstrasse
(2 stops) or changing trains to U 2 (three stops). However, a lot of buses
from the Netherlands park there in front.
The other is called Orion hotel in Dahlmannstrasse which is around 75 Euro
incl. breakfast and where some of my friends stayed and liked it. It is a
typical Berlin residential area with cafes and restaurants.
It is 5-10 min. walk to Adenauerplatz.
Both hotels can be booked via hrs.de.
May I point out that Staatsoper is having 'Festtage' under Barenboim
the week before Easter (Tristan, Eugene Onegin with Villazon and
Simone Boccanegra with Domingo) so you will be hard pressed for
choices.