Just checking the specs in more detail. They both have a Quad-core i7 processor but different models ...
Dell = i7-4702HQ, slightly lower power rating, speed-boost up to 3.2Ghz and 6Mb Level 3 Cache and HD 4600 (GT2) graphics
Mac = i7-4960HQ (I think), speed boost to 3.8Ghtz and 6Mb Level 3 Cache and Iris 5200 Graphics + 128Mb GPU cache
Edited to add cool comparison: http://versus.com/en/intel-core-i7-4960hq-vs-intel-core-i7-4702hq
There is a ~$300 difference in processor cost that easily figures into the prices so it isn't necessarilly more expensive for the Mac, you do actually get more as well as long as you look beyond the "Intell i7 quad core" sticker. Do Dell even still put those stupid Intel + Windows stickers on?!
The Mac also has the discrete NVidia graphics as well as the better integrated ones.