Next: the real cost of private offices.
You should include the cost of effective insulation/sound-proofing. (I've seen/heard it done reasonably well with expanding foam during wall construction.)
P.S. Hey, folks, the original author, Rich Armstrong (richarmstrong), actually replied in a response this comment, but you killed it off -- it's dead, now.
http://news.ycombinator.com/item?id=2868730
Maybe a mod could unkill it? Seems unfair; it was a perfectly legitimate response. I'll copy/paste it (presuming permission):
I'd love to do that, but our offices are glass-doored, so not sound-proofed, though that's a good idea.