At less than half of those prices (real hearing aid prices quoted above), you could get a top of the line mobile device, then search the associated app store for "hearing aid". There are already plenty of apps for free, $0.99, $1.99, etc, that take input from the built-in microphone and pump it through to the ear buds.
Not the best quality, but it would also function as a music player, video player, e-mail client, Twitter client, web browser, handheld gaming device, video chat device, voice memo recorder, GPS/map, electronic book reader, telephone (if that's the kind of device you chose), scheduler, TiVo remote, sketch pad, musical instrument, Victoria's Secret catalog, virtual pet simulator, news aggregator/reader, alarm clock, phone book, calculator, units conversion tool, dictation device, data organizer, note pad and outlet for paid entertainment content.
Except that apps sell for so damn little these days... It would be hard to get rich, even with a kickstart. It would probably be better to sell an add-on external microphone / app combo to turn it into a decent hearing aid. Early turn-by-turn navigation systems were external hardware / app combos too.