Here's my train of thought. WiiFit (game only) seems to be a pretty low budget title (Similar to Wii Play, Wii Sports, etc). So I can easily see it being bundled with the Balance Board for $50.
Nintendo is (supposedly) already making money on the Wii Bundle as it is. $200 for the system and $50 for Wii Sports. I don't know internal Nintendo numbers or anything, but I'd say they could drop the price of the hardware by $20 and still make a small profit.
Add in the fact that a title that has already sold 9 million copies (Wii Sports), they've probably made their money back on it - thus if it were a regular title, they could drop the price on it at retail. Since it's a pack in, they'll have to drop the price of the bundle. Wii Sports (I think) is $50 stand alone in Japan, right? So let's drop it to $20, dropping the current Wii Bundle to $200.
Heck, part of me wants to say that Wii Sports could just be dropped from this bundle completely... but running under the assumption that it had a low budget and has more than made up for that by now, it just seems to make sense to continue to bundle it with the system, as popular as it is.
Now, add in the WiiFit and Balance Board for the $50 retail - and there ya go, $250. Way below the price of the PS3 and still below the (current) price of the 360 Core. Pack in two awesome and popular games (Which I have no doubt WiiFit will be) and two awesome controllers (Wii Controller and Balance Board) and you've got a bundle that would be unbeatable at Christmas.
Additionally, if Nintendo wants to ever see other Wii games using the Balance Board, they'll need to sell a *lot* of them. Packing them in with the system is the best way to do this.
My only concern with this if WiiFit is actually ready for release, though it did seem pretty far along. But if they get it out by Christmas, they could then reveal the secret Super Smash Bros, Brawl mode that uses it...