What makes a speaker thousands more...

Jeff's post is excellent. One thing I'll add, based on the experience of friends of mine who build speakers, is that the cabinets are a huge portion of the cost of speakers, both the enclosure itself and the veneers. As you get into speakers with superior bass response and non-resonant enclosures, the size of the enclosures and the bracing, etc. grow, and the price shoots up dramatically as a consequence. I think that less goes into internal parts than you'd think, unless the speaker manufacturer is willing to go a cost-no-object route.

I disagree with Abstract7 only in that, in my view, cables are the highest margin items in the industry (phono cartridges too, but they're an art in the labor, so I'll live with that).