The faster the signal processing and the more automatic features a hearing aid has, the more expensive it tends to be.
The faster and better the hearing aid is able to filter out background noise, the more expensive the aid tends to be.
The better the hearing instrument is able to focus on the primary speaking voice in a noisy environment, and the better the aid follows the voice as it moves through a noisy environment, the more expensive the hearing instrument tends to be.
Accessories such as remote controls, Bluetooth circuits and remote microphones add to the cost of hearing instruments.
The smaller the hearing aid, the greater the cost tends to be, due to the expense of research and development for miniaturizing circuits, and the greater technical expertise required for manufacturing.