the size and accuracy needed for the strain gauges themselves makes them probably around 20-30% of the cost of the unit.
then the engineering, manufacturing and other materials that go into making it...
human beings don't produce a terribly large amount of force on the pedals (figure between 0 and 1000 newtons assuming a range 100-1400W of 'power' output)
load cells of that size can be $80-600 depending on how accurate you want them.
the higher the accuracy of the reading (which would be necessary to get the finite changes of 1-2W of power change).... the more expensive they get.
also, niche market for the most part.... figure average bicyclist is never going to have one... let's take the US as a small example...
last year there was a reported 11.something million bikes sold with 20" or larger wheels...20% of which were road bikes (primer power meter market)... or 2.2 million or so, even if 5% of those bikes were to riders who use power meters... you're only talking 110,000 units on an annual basis... not a huge market at all, as such... high cost.