In the US the Xbox is *currently* outselling the GameCube. I'm not sure if they've managed to catch up to the total GameCube sales though as until the price drop the GameCube was outselling the Xbox. In Japan and Europe the GameCube is outselling the Xbox.
However the PS2 is outselling the two of them combined. And has a huge lead. THe PS2 has won this generation. Nintendo will probably come 2nd and MS 3rd.
Nintendo (or MS too I guess, it is still really early) might be able to pull off a respectable 2nd because hardware doesn't matter. Most people don't care their system has 6.4gig/sec of bandwidth and vertex shaders (hell, some people on this board though their computers could play console games!). Which system has Tony Hawk, Grand Theft Auto 3, Gran Toursismo, Pokemon, Mario games, etc? That is what matters. If Nintendo can release software to drive demand they may come back.
Originally the GCN (GameCube Nintendo, NGC was already taken by the NeoGeo Color portable) was 66% the price of the other systems ($200/$300), but now it is 75% ($150/$200) the price. The GameCube was designed with price/performance in mind. Xbox and PS2 have automatic loading trays while the GC you have to snap the disc in place by hand. Heck, just pick all three systems up. THe GameCube feels like it is an empty plastic box compared to the other two systems.
THe original manufacturing costs for the GC were estimated at $180 (Nintendo still lose $20 by the time each console was sold), but you can be sure they've come down. If NIntendo can get their system down to just $99 this Christmas and can show a couple great games like Mario Sunshine they may catch or beat Sony's sales (but I doubt catch Sony's huge lead).
Finally Apple had nothing to do what so ever with the GameCube. The Gekko processor is basically a G3, but it has "pair single" floating point instructions, not quite Altivec, but still two floats can get processed in parallel instead of a single double. There is also a couple instructions of prefetch cache control I don't belive the standard G3 has.