I had my ears damaged by guns in the military and am close to deaf however I can also hear lower frequency sounds better than high (women and kids).
When the ear test is performed it measures each frequency range and then the hearing aid is programmed to produce a flat response based on each frequency range. As an example if you have a hard time hearing 8kz then that frequency would be amplified more by the hearing aid.
The newest hearing aids are very small and fit entirely in the ear (some even permit you to shower with it in). The technology has greatly improved as far as battery life, ease of programming, and as far as size decreasing so I realize that the cost needs to go up... but my last pair were $5,000 each
In actual material value there is probably $20 worth of material in it so there is a huge profit going to the company and the audiologist... but then there are not all that many people who are hard of hearing.