According to iLounge, who's generally reliable as far as Apple rumors go—the adapters that will make the iPhone 5's smaller dock connector work with your 30-pin speakers and such will cost 10 bucks a pop, or three for $29. The future don't come cheap!
The pricing, if true—and if there's even a new dock connector in the first place—is right in line with Apple's $10 MagSafe 2 Converter, which sticks one magnet onto another magnet so that you can charge your current generation MacBook Pro.
Why a three-pack? Because you probably have multiple things that you plug your iPhone into currently, and it's easier to spend 30 dollars on adapters than it is hundreds of dollars on new speakers and such. Also, these things are going to be ridiculously easy to lose.
But hey, all in the name of profit progress, right? [iLounge]