While perusing Amazon today I noticed that the two player co-op shoot em up, Deathsmiles, is on sale for $22. More specifically, the limited Xbox 360 edition of the game which normally runs $49.99 is only $22. Limited Edition extras include an exclusive Deathsmiles Xbox 360 faceplate, a CD soundtrack and special boxart.
If you're looking for a different sort of SHMUP which showcases something other than space ships, Deathsmiles might be just for you.