What Is The Cost Effectiveness of Geothermal?

The colder the climate is at your location the less economical geothermal becomes, do to heating requirements.
The warmer the climate is at your location the more economical geothermal becomes, do to cooling requirements.

Either way if you install a geothermal heating system it will be the largest consumer of electricity in the house.

As an example it would cost about $1,430.00 a year with an effective "delivered" rate for electricity of thirteen
cents per kilowatt hour for the 11,000 kilowatt hours of electricity that the Canadian GeoExchange Coalition
says is the average consumption of electricity for an average residence (don't forget to add taxes!).

Geothermal heating systems are increasingly becoming more economical for both residential and commercial (much more for commercial with the larger neeed for air conditioning) applications, however with the current costs of energy, they are not as economical to operate as conventional natural gas heating system (forced air furnace, boiler, etc.).

Using geothermal energy to cool the building in the summer dramatically improves the system's economics and
its environmental impact, compared to an air conditioner, which consumes considerable amounts of electricity,
if it is in steady use, compared to the electricity required by a geothermal system for cooling!
Although there are in fact very few days of the year when air conditioning is actually needed, especially with a modern well constructed building or older buildings with upgraded insulation, energy-efficient windows, window coverings, etc.

It is foreseeable that there will come a time where the cost of energy and the cost of operating a geothermal heating system will become more economical. We see the most practical applications right now as being commercial and rural residential, where with the cost of bringing in the utilities is a factor, if you are only required have power!

Installation cost versus the size of geothermal system?

Like a conventional heating system a geothermal heating system should be sized for the heat loss of your building.

Systems should be designed to meet the building's heat loss, systems are sometimes sized to 70-80 per cent of
the heat loss (which should meet heating needs on all but the coldest days), to considerably reduce the installation
costs of the system. An undersized system will have to work too hard (consume more energy) to heat the building. 

Ensuring your geothermal heating system is designed to provide the optimal results will likely result in increased installation costs, but effectively reduce energy consumption and operating costs.

Of course, the required size of any heating system and its installation costs, can be
considerably reduced if your building is well insulated and you follow good energy conservation practices.

The CSA standard  C448 for installation covers the design of geothermal systems.