On my sscx build, aside from the frame ($450) the wheelset was the most expensive bit at $175 on sale. The King headset was a close second place.

On my beater ss roadie, the B-17 costs more than I spent on the wheel components, but someone gave it to me so I can't count it. I guess the $98 wheels were the most expensive part. The rest of that bike (frame included) is dumpster and garage scrounged parts.
Right now, the Contour HD camera I've got under the handlebar is the most expensive thing on it.