FWIW
I have to weigh in with Charles. The part may cost 10 cents to mold, but setting up for consumer sales is costly. In a previous life at a Motorola manufacturing plant, I was told (and saw how) a $3 transistor would be +$30 by the time it got into the technicians hands. Most of that was "handling charges" incurred throughout the manufacturing, packaging, inventory keeping, transfer though at least four different warehousing stockrooms, etc, etc, etc. Unless you really see the process it is pretty easy to have the "can't cost that much" view.
Besides, selling replacement parts would curtail X10 product sales. And that would be anathema to X10.
Periodically these switches fail when the light bulb burns out. Save the defective switch for parts scavenging such as this.