I've said this several times in the past, you get what you pay for.
X-10 is cheap precisely because there are faster; more reliable options out there. Z-Wave is more expensive, yes, but you get:
(1) confirmation that commands or events sent were sent correctly, if not, the originating device tries again.
(2) faster data transmission rate, even the oldest Z-Wave revision communicates at 9,600 bits per second, opposed to 150 bits per second over X-10. This becomes more noticeable as larger scenarios trigger more devices.
(3) X-10 is _very_ susceptible to transient disruptions on the power line. A refrigerator, or a washer/dryer can literally cause X-10 devices to spontaneously turn on/off. It requires literally a phase coupler and a filter to try and mitigate the problems. Z-Wave side steps this problem entirely, by not signalling over the mains, and INSTEON deals with this, by advanced error correction techniques. KNX, on the other hand, normally sends these signals on its own wiring (although RF and Powerline options are available there as well.)