While I think your project is a worthwhile project I do have a question about your cost calculations.
Under a traditional setup (PC & Scanners) you have "8 Uniden Scanners: $250". Does that means you're getting scanners at $31.25 each? Where???
My next question, are you using 1 RTL Dongle per frequency? If so does one Raspberry Pi have the power to handle 8 RTL Dongles and DarkIce to stream 8 audio streams.
Very interesting project which I'll be watching...
I was being conservative on the costs so that it didn't appear that I was trying to fit the budget into the goal. I've purchased lots of BC350 scanners on ebay before for under $20 a piece, usually from a police department that swaps them out or some other large agency. Agreed, though, it's tough to find them in quantity often.
It's custom software, written specifically for this purpose, and includes the streaming libraries (lame and shoutcast) natively in the program. The current configuration will handle 2 dongles per Pi, but we're using a powered USB hub to support the dongles, wifi adapter, etc. The hub is included in the calculation, though. So, if you could find the right mix of frequencies, you could stream 16 different frequencies from a single Pi and 2 dongles.
Alternatively you could stream 8 frequencies and run dump1090 for ADS-B on the other dongle, or even acarsdec. Heck, with 2 Pis you could do 75% of the ACARS frequencies simultaneously, ADS-B, and 8 channels of audio - for around $100-$125.
The software would technically work with MilAir, although I haven't tested it. It's running at Oshkosh now with good results - that's the first real field trial to date. If you're interested in trying it with MilAir, send me a PM.