Geothermal energy is forecast to be the cheapest form of renewable energy and is expected to be competitive with gas and coal when the cost of carbon is factored in. A recent study by highly regarded energy economists McLennan, Magasanik and Associates (MMA, 2009) has forecast the cost of electricity production at 2020 for a wide range of generation types.

The costs are presented in Figure 4 and Figure 5 and show that Volcanic, Hot Aquifer and Hot Rock geothermal projects are forecast to be the only sources of renewable energy able to produce at less than $100/MWh.
Forecast cost of geothermal versus fossil fuel energy generation at 2020 based on CPRS -5
scenario (after Mac Lennan, Magasanik and Associates)

Comparison of forecast costs of renewable energy sources after Mac Lennan, Magasanik and Associates)