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It's actually an 11% ammonia and distilled water mix, if I recall.

Problem is commercial (neat) ammonia sometimes isn't pure enough to use without building crystal deposits on the injector and substrate.

We used to mix our own in the lab. Cost $0.11/gallon to make.

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Kinda sorta. Diesel Exhaust Fluid is really 1/3 urea (a nitrogen compound that TURNS to ammonia when heated) and 2/3 distilled/deionized water. When injected into the exhaust stream (think "after treatment") into a catalytic converter-type burner it very effectively consumes harmful oxides of nitrogen from the exhaust.

The beauty of this system, while cumbersome, is that it's an AFTERTREATMENT approach. Why do I stress this? Simple--the engine doesn't know it's there. Period.
What does that mean? Simple--the engine can be tuned for max mpg and power without regard to oxides of nitrogen.
That's a beautiful thing.