A similar spec'd Dell was more expensive, at least in the few I put together.
In the UK, Apple with student discount (of 16%) is about the same as Dell on the T7500 (octo) so I guess Apple's mark up is around 1/0.84 or close to 20%.
On a four core, single processor, if you're willing to go with a Dell T3500 (smaller case and power supply) then the cost of the Apple is more like 50% to 60% more and the Dell provides the option of Quadro cards and a 3.2GHz processor and SAS drives without having to spend £500 plus on a RAID card. And most importantly (from my point of view) is Dell provides six RAM slots per processor to Apple's four (effectively three if you want to stick with triple channel memory access).
Personally I really like the Apple design and have been looking to buy a Pro for eighteen months or so but I really can't take the double wammy of paying a big excess and then on top of that having to buy very expensive 4GB RAM modules if I want 1GB per virtual core or more. (And I am a student so I would get the Apple discount).
I should add that for the 08 Mac Pro the situation was very different with Apple being cheaper than Dell even without the student discount - it is the '09 model that has come with a huge price increase. (I'm talking prices relative to Dell, or a DIY build as opposed to the absolute price which is more open to debate).