Here is the cheapest I found it, I think they changed the price from £17.99 recently.
* The fastest way to recharge one or more Rechargable Battery Packs
* A cleaner charging solution - no wires to plug into your console
* Always ready and easily accessible - no need to stow it after each use
* Charge one battery pack in less than two hours, or two packs in less than four hours
* Battery offers up to 25 hours of play per charge
* Includes charging stand for two batteries, one Xbox 360 Rechargeable Battery Pack, and a power cord
Recharge in a flash and kiss disposable batteries goodbye with the Xbox 360 Quick Charge Kit. Quickly recharge up to two battery packs at the same time. Your Xbox 360 Wireless Controller needs a new battery? Swap in a fresh pack from the Quick Charge Kit. Multiple wireless controllers? Pick up a couple of extras, so you'll always be ready with a fully charged battery that's ready to go. Includes one Rechargeable Battery Pack, additional packs sold separately.