If you are going to Barcelona and love classical music and/or good architecture, then the Auditorium of Barcelona is well worth a visit. It has a music museum, which has changing exhibitions as well as permanent displays, within it as well as being the main place for classical concerts in the city. There are plenty of budget hotels in Barcelona near to the Auditorium of Barcelona, which is conveniently located next to the national theatre.
The auditorium houses the Catalonian Academy of Music, as well as the Catalonian National Opera. The website of the Auditorium has details of all of the concerts that are going on there, and so it is advisable to take a look at this before you book. That way you can find a cheap hotel near to the Auditorium of Barcelona in Barcelona, so that you can reserve tickets to the concert before you get there and be sure of your seat.