Hearing aids cost so much because the various cartels that make them got them classified as medical devices decades ago. There are all sorts of legal and regulatory hoops you have to jump thru before you can call something a “hearing aid”.
You can buy a bluetooth earpiece for R300 that has the exact same parts – condenser mic, speaker element, battery, and opamp/EQ circuit – and has vastly more functionality, including the bluetooth radio system and spiffy LED indicator lights.
Hearing aids are configured with an equalization curve tailored the the wearer’s specific hearing loss, but it’s not like there are a million different kinds of loss. It’s mostly “top down” according to age and environment.
Only newborns can hear 20khz. We lose a few thousand hertz before we hit puberty and pretty much everyone loses everything above 12k by their 30’s. (by “lose” I mean response is down a considerable number of decibels from our factory abilities).
Impact-type noise from construction, artillery, or rock bands can punch holes in what’s left, especially in the voice frequencies, but it’s not like it’s DNA-complicated or something.
A simple hearing test can identify your remaining response curve in a few minutes and it isn’t going to be that much different from the guy on the next bulldozer on the left or the guitar player on the other side of the stage.
There’s no reason an ear specialist or audiologist couldn’t give you a “prescription” response curve when you go in for a hearing test that you could load into a device that costs thousands (tens of thousands if you want it to be super small) yourself with an app of some kind. The 4-5-figure hearing aid price tags are simple price gouging by a “medical” cartel.