Very good article comparing the price of a Mac (all the various models) with a comparably equipped PC.
http://technologizer.com/2008/08/14/are-macs-more-expensive-lets-do-the-math-once-and-for-all/
A few things I like about this article, he chose a middle of the road Mac for each model type, he picked 3 different vendors to compare with for building the PC versions. He also answers the most common questions about how he went about his test, and also the details of how the various machines were equipped. They all couldn't be EXACTLY the same because there are some differences between the machines in what's available.
Summary (if you don't want to read the entire article, but I recommend you do, it's got a lot of good information)
Macbook - Comparable in price to the PC versions (less than $100 difference)
Macbook - Base Model, stripped PC laptops are cheaper (I disagree with some of his points on this one, specifically battery life and performance, giving an advantage for including works is questionable) btw this is the version I have, and the biggest issues he pointed out HD size and ram, I was able to upgrade VERY cheaply.
The next sets of comparisons will be interesting, stay tuned :)