MUPA is the comparatively new concert venue in Budapest, less than ten years old it nevertheless has among the finest acoustics in Europe. It has two concert halls and a very active calendar in both (though less active in the summer), including two major world-class symphony orchestras resident and many others that pass through. There is also a modern art museum, an excellent arts bookshop, a restaurant, and grand spaces indoors and out.
On the day of performance, anyone with a student ID (from anywhere in the world, I'm told, not just Hungary or the EU) can get standing room tickets to any MUPA concert for 500 Forint (about $2); the venue (or at least the ushers) don't discourage the standees from migrating to vacant seats once it is clear that those seats are not going to be filled.
Tickets become available one hour before the concert: