I think they base it on 4-6 hours of work. That would be $300-$450 at a shop rate of $75/hr.
I did my own so that my labor savings bought a new flywheel (vs resurface) and my new firewall adjuster and quadrant. Overall I spent right at $500 for clutch, FWA, quad, cable, flywheel, and some new bolts for the flywheel and starter. Starter bolts were worth it. They are a 12pt head and it allowed me to use a ratcheting wrench to change my starter alone in about 30 minutes.