Are you familiar with LibriVox? This is a website with public domain audiobooks available for downloading. For free. Can’t beat that!
LibriVox represents the work of an army of volunteers: According to their website, “LibriVox volunteers record chapters of books in the public domain and release the audio files back onto the net. Our goal is to make all public domain books available as free audio books.” I’ve heard that the audio / reader quality can vary, but most people seem quite satisfied. The catch is, of course, that LibriVox only has materials that are in the public domain in the United States, generally meaning items published prior to 1923.
If there aren’t worries about particular versions or translations, LibriVox could also be a wonderful alternative for your students, whether they are looking to save money or whether an audio format is simply easier for them to use.