When the Xbox was launched MS took a $155 hit on every console...
(it cost $455 USD to manufacture ship etc.)
Today MS still takes a hit but only about $52 (so today it costs them about $202 to manufacture ship etc.) Which is substantially less however it's still a loss
By comparison Sony MAKES $48 on every slim PS2 sold.
The biggest cost savings as mike said is in royalties.
Sony owns all the tech in the PS2 so they pay NOTHING in royalties (well except maybe rumble now but...

)
The Xbox probably had to pay royalties to Intel for the PIII, nVidia for the GPU, Sony and the DVD consortium for DVD playback and DVD formatted discs, more royalties for whoever owns progressive scan/ HD etc.
This is why the xbox was so "broken up" and also why the 360 will be broken up as well. The Xbox was so front loaded with Royalty costs for the CPU and GPU alone that the everything else was sold EXTRA.
for instance... the cost of the DVD video playback royalties were part of the DVD playback kit's cost. the cost of progressive scan and hd video playback, and surround sound were built into the cost of the Advanced AV pack (which is why consoles use a custom AV port).
We're getting some of this stuff built into the Xbox360 but really only the techs that have come to maturity and are available for "public use" without royalties. such as DVD video playback and progressive scan. (things like this are part of the reason they are able to drop the price of the console as time goes on).
MS was really smart this time around and they own just about all the "rights" to everything in the 360 so there will be little to no royalty costs involved. The royalty costs would however come in the format of things like the wireless networking and DVI/ HDMI ports, which is why those will most likely not be included... so they can break out the cost of those royalties into the add-on devices.
The sad part is... I think the biggest royalty payment MS has to make in the 360 is to nVidia for BC
