Don't get me wrong, I love my wii. But outside of first party games (SSBB, SMG, MetriodP3, MarioKartWii, ZeldaTP (which was really a gamecube-turned-wii game)), i NEVER play it.
The point in saying that is not to bash the wii, but rather to show you that it is not truly a gaming (traditional definition) system. Only Nintendo is making quality games for the wii and that really lessens it's appeal as a gaming system. That being said, it has never been marketed as a pure gaming system. It has been marketed as motion-entertainment.
What i'm getting at is that, try as you might, you really can't compare the wii to the 360 or ps3 because they are just not the same. So what if the 360 cost less than the wii? A significant portion of wii costumers (non-hardcore-gamers) would never even consider a 360 anyway, whereas most potential ps3 costumers have probably considered a 360 as well due to the existence of multi-platform games.
It comes down to this simple fact, $249 is the price for this motion-entertainment system (not just a gaming system), and until another company can release a product in this genre that fills this niche, the wii will hold its dominant monopoly at whatever price-point they desire.
Nintendo has succeded greatly, but as tindytim mentioned, it is due to the fact that microsoft and sony has remained adhered to the genre.