I don't see the price tag having any impact in the iOS/OSX software ecosystem, so, why are they bothering to charge for it?
http://www.akamai.com/html/about/press/releases/1999/press_0...
It was at one point worth about 2 billion dollars, but I'm guessing it all got washed out after the dot-com bust because word of it was never mentioned again. :-)
Anyways, they aren't doing this because of CDN costs, it's negligible -- they are Akamai's largest customer and are also building their own network as well.
I am quite sure the number of Mac users who download Xcode is negligible compared to the number of Mac users who download movies from that very same CDN. And the size of Xcode is comparable to HD movies.
My guess is that the price is low as a way to introduce people to the idea of paying at all. The small price precludes an outcry. Once everyone is used to paying, they can slowly increase the price over time.
If they started by charging $50 or $100 or more, I suspect there would have been a huge outcry. But I also suspect the price will get to that point sooner or later.
Also, now that the platform has been strongly established, Apple can charge a price for the development tools without fear that developers will abandon them.
Though I really don't think it's a good idea to make Xcode 4 non-free.
In the grand scheme of things, Apple's tools and developer membership schemes are fairly generous. It's still making a big step from free to non-free as others have already said. I'm not going to rush to upgrade.