Exactly. It's more than just the $5 for the app and the data cap/bandwidth issues. It is wear and tear on a machine that has to be left on 24/7. It is the hydro cost of running that machine 24/7 instead of turning it off when you go out (this alone may well add up to more than $20/year!) If you don't want your main machine on 24/7 then it is the cost of another Mac mini or NAS or other device to act as the server instead.
It is also wear and tear on your 2 TB drive that has to be on 24/7, as opposed to working more like a backup drive that's only activated occasionally to back up your music files. It is the hassle of ensuring AudioGalaxy and your server and your ISP internet connection are all up and running when you need them to be (dealing with power outages, internet outages, maintenance, restarts, software updates, etc.)
$20/year might well be worth it for the uptime and hydro considerations alone.