I've done a bit of work in hearing aid development (master's), and there are a few issues that make them so expensive. Most decent (digital) hearing aids are not just simple audio gain. Aside from possible loss from occlusion (which can be fixed surgically) hearing loss normally doesn't happen in all frequencies equally. Sometimes you can get away with a simple gain hearing aid, but it's not really matching what your loss is. As mentioned above, the gain has to be adjusted in bands, which requires a dsp capable of performing banding and gaining in real time so the lip sync doens't get off. Now, this doesn't seem like a hard task, but have you ever seen the size of the chips they use in hearing aids? Compare the size of a simple BTE aid and the space it has for the hardware. The newer ones are fully hidden, making them less than the size of a finger. If you're ever seen the chips, they're maybe the size of a few pin heads, if that. And, they have no fans to help heat dissipation and the power comes from an unbelivably small battery. It is NOTHING like a computer. If you can fit a computer chip and board in your ear, more power to you, but most people can't.
Not only that, but many digital aids also include noise reduction, because background noise is a big problem - it's amplified directly into your ear. Plus, there's feedback. A hearing aid can be designed to be open, so you need to repress the feedback, or closed, which has it's own problems - ie, it's forming basically an echo chamber in your ear. For example, stick your finger in your ear. Sound strange? Same thing with a closed aid, only now imagine your finger is talking. So there's that, and the sofware development costs that go along with it.
Not only that, but you have to pay a trained audiologist to fit the device physically, and do the appropriate band gain settings and whatever else the hearing aid needs.
Anyway, to sum up, they're expensive because:
- hardware costs - very small size, low power, low heat
- software costs - banding, noise reduction, feedback comp
- audiologist costs
- possible licencing costs (wasn't too involved in that end, so I'm not sure)