You generally get a custom mold for your ear canals and a specially tuned DSP for your frequency curves + BT connection to your phone for calls, at least. Your device can be retuned over and over as long as it functions, too.

What drives the prices up is a multitude of factors: High end DSPs, micro speakers which can do good sound reproduction at required frequencies, relatively low sales volume, R&D expenses and of course an insatiable appetite for profits.

These things always cost and arm and a leg in here, too.