1 GC pad for 6-7 quid ish? Maybe 2 pads.
I agree with vonhelmet, and doubt most Wii owners download anything from the virtual console (which somewhat explains the criminal lack of support for Lost Winds!) and need anything more than a couple of Wiimotes/Nunchucks. Unless you have an army of kids who all demand a motion plus, peripherals for the Wii won't cost much. 2 pads and the wireless adapter for the 360 cost me more than the extras purchased for the Wii.
Indeed, unless you have a family with lots of kids i can't see the majority of wii owners having 4 of each, unless you really want to be prepared.
As you say for the 360 the wireless adapter costs £60 which is very expensive (ironic the wii has this functionality built in and a Microsoft console doesn't mmm).
I'd quite like to see the prices for Natal and Playstation Move when they're released. I bet that will add up fast for the equivalent functionality.
(Own and enjoy all 3 main consoles btw, before fanboys pounce)