PMs work with starin gauges and bathroom scale work with strain gauges.
I have a bathroom scale the gets down to .01 lbs and it cost me $70. I have a PM that measures down to the .01 ft/lbs and cost $4000. One was manufactured in Germany and one was manufactured in Asia. Granted, the PM does velocity as well but so does a $40 cycling computer.
My point was/is....... I think a significant amount of the cost of a PM is (obviously) not technology, materials, or engineering.............. it's labor. I dare someone to tell me it's the cost of the programing and software
My iPod has umpteen more MB memory than my SRM headunit and cost .02 as much. A Garmin can (all be it not as effective) be used as a headunit for ~$300 while a PCVI costs $1100 is you buy the HRM strap and the Pod. By all intents and purposes, an iPhone may soon be able to serve as a headunit.
Why do PMs cost so much?
Because they can and people will pay for it.
I seen the one legged argument and until you can prove it you blowing smoke. In fact, there is a well know power coach, with one leg, that race and trains with a PM. Because the crank is a system and energy is transmited via the spindle................. it's a single system.
Accuracy is not the problem.
Additionally, the muscle "imbalance" argument is trite at best. Recently studies from the UT and UUtah have determied that imbalances are caused more by biomechanics and not actual muscle composition or useage. If the latter were true we'd walk in circles and not beable to control it. Moreover, trying to correct a slight muscle imbalance is a futile exercise as you'd need to balance it in every
part of your life that involves useing that muscle group. Further, this does nothing to express imbalances in other sports: XC skiing; speed skating (left turn only); NASCAR (left turn only with no power steering); Golf; football/soccer; etc. I have yet to find out why cycling is some much different than any other sport?
Moreover, what does a imbalance have to do with where a PM is manufactured? A PM is not even the best tool to do determine an imbalance.