There is an incredible variety of power meters now on the market, which has helped their popularity, volume of sales and therefore helped bring the prices down. However, it's still very much a developing technology, which has to be accounted for in the end price.
From our experience, it's a 'you get what you pay for' product. The best power meters will have invested a significant amount of research and development in ensuring that the end product is consistently accurate, robust enough to withstand forces and harsh environmental conditions and to develop the supporting software.
Entry level - typically £250-£400/$300-$500
At this price you are likely to get a single-sided measurement that doubles the reading to estimate both legs. Stages 105 cranks are available at this price, as are 4iiii and the outgoing Garmin Vector 3 single sided power meter.
Mid-level - typically £550-£800/$700-$1,000
Mid-price starts to see pedal-based systems and dual-sided meters. Crank and chainring systems also become available. Professional athletes are increasingly using meters at this price point. Favero Assioma pedals are a fine example.
Top end - £900/$1,200 +
At the top of the market, we find dual-sided measurement and crank and spindle based systems. Examples include the Rotor 2InPower and SRM.