At its most basic, an audio panel is just a series of switches that direct different audio sources to your headset. You can accomplish the same thing with a set of discrete switches. They are pretty simple and can be very cheap, and I'd be willing to build my own if I was working on a very light/small/cheap experimental.
Busy that's just for the basic functions of switching audio sources. The audio panels you see for sale, even the most basic units, have a bunch of additional options, like built in intercoms, audio recorders, crew isolate, Bluetooth, XM radio, etc. That's what makes them so pricey.