The expense is usually due to the physical hardware needed.
Somewhere along the way some device needs to measure the power output.
Well inside the hub seems like the most common version. Thus you need a wheel build around a 'heavier' hub to get this to work, and thus is never cheap.
Polar had a power sensor I never could figure out how it worked, but it had an external sensor along the drive train side back stay. It supposedly watched the chain tension, on top of cadence and wheel speed to calculate power. But that silly sensor was close to $500 or more at the time I looked at it. Still seemed like Voodoo to me, personally.
A friend relied on it, and thought it was pretty good and accurate.
ANT+ is just a data transport protocol, which makes the send/receive module cheaper and more common, but for power, the rest of the sensor is the expensive part.