Finally a company has released a compact bicycle car horn. Sounds identical to a standard compact car at 112 db, with high and low notes, all for under $100.
My custom car horn installation, with 2 RadioShack 7.2 V NiMH battery packs, cost about $150, plus my time, to macguyver and install. I like how much smaller the built-in USB LiPO batteries make this powerful horn, all with 30 seconds of continuous honking possible, and 1-2 months between recharges.
Check out Loud Bicycle here: http://www.loudbicycle.com/