Hydroelectric energy is inexpensive. In the US, it costs an average of 0.7 cents per kilowatt-hour (kWh) to produce hydropower. Hydropower costs one-third the amount of fossil fuel and nuclear fuel, and one-sixth as much as natural gas.
Hydroelectric energy is pretty inexpensive. The initial start up cost of building a damn and a power plant to harness hydroelectric energy can run in the millions of dollars. You can find more information here: http://en.wikipedia.org/wiki/Hydroelectricity