http://claverton-energy.com/active-power-article-flywheel-en... as a smaller scale example. $330/kW but at that price it only has 15 seconds of carry-over, basically just enough to get the diesel generator fired up.
Spinning reserve in the grid is equipment that capable of long-term generation very quickly. In the case of hydroelectric dams, they will often cut off the water supply to some of the turbines and use air pressure to push the water out of the way; the generator attached to the turning essentially turns into a motor and keeps the turbine spinning. If you need to bring it online, you open the water valve and let the air out.
Similar situation with natural gas-fired simple cycle turbines. They’re sitting there running at low output. Need more? Just add fuel. For combined cycle it might take a bit for the boiler to warm up for full output but having the first stage running full tilt will get it warmed up fast.
