Amazon.com has some fantastic deals on MP3 album operas. These are from the label Past Classics, and are very old recordings of varying sound quality, and for the most part are only split into a few tracks, and of course no libretto. However, considering the cost of opera cds, and the fact that you can download many librettos online, these are a great way to expand your opera collection. These all cost $2.97 or less. Do be sure to listen to the previews to get an idea of the sound quality. These recordings feature great artists from the early part of the last century like Maria Callas and Tito Gobbi.

Zubin Mehta and the Israel Philharmonic in an excellent disc of 6 Liszt 'Hungarian Rhapsodies' arranged for orchestra. On CD or MP3 Album.

Philips Trio has packaged Mozart's compete String Quintets featuring the Grumiaux Trio with guests in a great 3CD set. These are about the best set of these works you can find, and at a bargain price, and the MP3 version is an even better buy.

Murray Perahia recorded the Mozart Piano Concertos in the 1970s and 80s and the set has been remastered by Sony. The 12 disc set is a fairly good deal on CD but is a great bargain on MP3. These performances with the English Concert are highly recommended.

Updated 6/2/10: I do not usually recommend large classical box sets because they are expensive and not always a good buy. However the new '111 Box' by Deutsche Grammophon is a fantastic buy. The set is a 55-CD Box of full albums from DG's first 111 years. The box set seems to be out of print now, but the MP3 version is available for around $108, so a fantastic bargain. DG has set up a special site at dg-11.com with all of the information on the albums, where you can listen to a full album from the collection every week.
If a 55 disc set is too daunting, there is a 6 disc set with one track from each of the albums in the 111 set, plus other tracks for a great price too.