According to iLounge, who's generally reliable as far as Apple rumours go—the adapters that will make the iPhone 5's smaller dock connector work with your 30-pin speakers will cost 10 US dollars a pop, or three for $29. Knowing Apple's dollar to pounds ratio that'll probably end up working out around £10 each. 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 £9 MagSafe 2 Converter, which sticks one magnet onto another magnet so that you can charge your current generation MacBook Pro/Air.
Why a three-pack? Because you probably have multiple things that you plug your iPhone into currently, and it's easier to spend 30 quid on adapters than it is hundreds of pounds 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]