If making your own cleaning solution presents a really strong draw for you, have you considered wet tumbling with stainless steel pins? Home brews work much better there, since the mechanics of the actual cleaning operation are simpler.
Ultrasonics work via cavitation. The waves sent out by the transducers create tiny vacuum pockets in the solution. The cleaning action takes place when those tiny pockets collapse and the liquid rushes in to fill the void. The collapse is physically violent and generates momentary temperature spikes that can approach 5000 degrees. The aggressiveness is controlled by the frequency of the waves. Lower frequencies create larger vacuum pockets that carry more energy. Higher frequencies create smaller pockets that can form in smaller spaces that are also gentler when they collapse. So, what does this have to do with home-brewed cleaning solutions? Things like the concentration of dissolved gas and ionic qualities change the cleaning solution's ability to form vacuum pockets (this is also why most solutions work best at about 80% of their boiling point). Furthermore, you don't want to use any chemicals that might embrittle your brass or cause stress corrosion cracking. So, personally, I prefer to leave those types of details to chemists who actually know what they're doing.
As you noted, the commercial concentrates designed for cartridge brass cost about $20/quart. I don't have a bottle in front of me, but I think the Hornady solution that I use gets diluted 30:1 or 40:1. At that level of dilution, the cost of the cleaning agent isn't a whole lot more than the distilled water it gets mixed it with. I like my brass really clean, and a quart still lasts me a long time.
For wet tumbling with pins, lots of people have reported having good luck with Lemishine (a variant of citric acid) and Dawn dish detergent. I don't know what 10 gallons worth of that would cost, or how long it lasts before the solutions need to be replaced/refreshed.