The cost is significant. A low estimate is $3,400 per kilowatt installed. Geothermal energy requires a larger upfront investment. You must first secure he rights of the property, drill, conduct testing to see if the ground is going to be hot enough, drill again, set up the plant and then see how much MW you get from it. Just testing the property can cost millions and it might not be a viable source for a geothermal plant. This process takes years. The cost will end up in the 10's of millions before you see any results. Also once they are built, plants have a 45 year life span before needing repairs.
An issue with these plants are not that they are not very profitable. Without the tax incentives that are given to the green energy sector, investments would not make economic sense. This is especially true for the solar power industry. The technology needs to be improved to get more from less. This will happen with time.
Geothermal energy is limited to geographic areas where the ground in hot enough to produce a level of energy that would be worth the investment. For this reason I do not think it will be sufficient to cure the world's energy needs, however it's one step in the right direction. This technology in addition to other green forms of energy (solar, wind, geothermal, hydro) will help this country's economy and the environment. Oil is only a temporary solution to the worlds energy needs.
There are several publicly traded geothermal companies that are in the development phase (meaning that they do not yet generate any power). If you were to look at their websites and shareholder reports you will be able to get an idea of how much these companies are spending.
Nevada Geothermal
http://www.nevadageothermal.com/s/Home.asp
Western Geopower Corp.
http://www.geopower.ca/
U.S. Geothermal Inc.(currently producing power)
http://www.usgeothermal.com/