I don't remember the exact numbers, and this was a comparison based on the C2D MBPs with Dell laptops when C2D first came out, but you'd end up paying $400 less or so on the Dell then you would for the MBP. The difference, to me, was the Apple operating system, the better feel and construction (though I'll concede that may have been untrue), and simply the fact that I could run any OS I wanted on the MBP through Parallels/Boot camp, where thats not as easy on a Windows laptop. In the end, I felt like I had gotten a better machine through Apple than the cheaper Dell would have given me. Plus, that Dell probably sells used now for a lot less than I could sell my MBP for.
And it also has to do with product cycles. Dell and the other manufacturers constantly update their products. Its hard to go build a computer on dell.com and have it be the same two weeks later. Apple's products are the same usually around 6 months at a time. So, as Dell continually phases in new parts and products and configurations, they change the pricing on older parts, while Apple will still be using the same product a few months down the road.