It seems that commercial flywheels are about 7 times more expensive than batteries per kWh. The article mentions 24 batteries at ~$1,500 each providing "3 days" of power at an average of 2500 watts, so 180 kWh for $36,000. A recent blog post regarding Beacon Power flywheel systems quotes them at ~$1,400 per kWh, or ~$250,000 for 180 kWh.

http://neuralenergy.blogspot.com/2009/06/flywheel.html