Dell charges $1-2 per crapware install, but sometimes those are free if Dell feels like they add value. Vista Basic, OEM edition, is probably $50 (I am guessing about this one). This requires 20-25 cases of infection with useless shit to offset Microsoft tax.

This still can't explain how come Ubuntu-based machines are more expensive.

I think that Dell simply rides the demand train: they're the only big-name brand that sells Ubuntu-powered machines (very few know that Lenovo sells SUSE) and they're enjoying higher profit margins on them.