Depending where you look. You can find it for about a tenner cheaper. The price difference certainly isn't enough to totally sway buyers though, and it clearly the systems that they're buying. That is, a £50 XB360 wouldn't outsell a £150 Wii, because the market for the Wii just won't buy a 360.
I was really just trying to answer the question about Core pricing - it's been months since I've seen the Core (new) in shops, though I've seen them lingering around like the "bad kids from the council houses" online.
As you say, I don't think people who exclusively want a Wii will be swayed at any price (although those that want both may jump for the 360 first if it was cheaper). For the most part, a cheaper 360 Arcade would only appeal to people who wanted a PS2 or Xbox successor, but who didn't want to spend £200, and certainly didn't want to spend £200 (probably more than they paid for their PS2 or Xbox) on a functionality reduced 360.*
*I'd guess this covers most people that this generation has yet to reach!