Gaijin Games has a huge sale going on right now on a bunch of the Bit.Trip games over on Steam. Bit.Trip Collection (Bit.Trip Beat, Runner, Core, and Void) is going for $11.98, and that includes the soundtracks for each game. Or you can buy each game individually for $2.49.
Even Bit.Trip Presents... Runner2: Future Legend of Rhythm is on sale for $11.24. Or you can get it for $14.99 with the soundtrack.
You have until May 20 to score all these rhythmic games on the cheap.