Keep in mind that those are retail prices for early adopters of new technology. Price in this cases has very little to do with cost.
Not that I think it is likely, nor do I really even care, but if MS wanted to incorporate HD-DVD into a v2 release of the 360, I doubt it would cost them more than a few tens of dollars, at the most. Maybe less if you assume that the cost of supporting logic chips, drive assembly, motors, etc. are about the same cost for DVD and HD-DVD and the only significant cost increase is for the laser and tracking mechanisms. Maybe $10 increase in 360 manufacturing cost.
Of course, that's just that much more money that they would lose per console on a console that already costs them a decent amount for every one sold. But still, it's not like it would cost them $500/console to put in those drives.
In any case, makes little difference to me. I don't think any games would ever support the increased storage capacity, and a console would never be my primary high definition media playback device.