Installation costs will usually be higher for a straight horizontal system as this consists of more trenching than the coil (slinky) horizontal system. But on this account, the cost of a horizontal system will be less than a vertical one.
The cost for the heat pump is estimated around $1,500AUD per installed kW which adds up to $9,150AUD when matching the cooling demand of a typical 100square meter house in Melbourne (6.1kW* $1,500 = $9,150). Furthermore assumed that the compressor will need replacing after 12 years it is an approximate extra $5,000AUD.
Since this heat pump replaces air conditioning and electricity, costs for those appliances can be saved which is estimated at $5,000AUD every 10 years.
The running cost for a GSHP system mainly depends on the associated fuel cost, in this case electricity and usual tariff rates normally apply (pricing assessment recovered from ATA Heat Pumps and Environmental Performance Dec06).

Overall, this systems can save up to 50-60% on heating bills and minimizes the overall environmental impact over time. Having a GSHP installed will negate the need for the separate purchasing of heating and cooling systems as GSHP system is capable of both heating and cooling in one unit.
For a comparison, a normal an oil-fired boiler will cost much more to run and electric heating will be three times more expensive. Keeping in mind that gas prices are set to rise in the future, and heating and cooling systems generally need regular servicing while the GSHP doesn't.

The initial installation cost will be a lot more than a conventional oil or gas fired boiler but this initial expense is then covered by the lower running costs, lower maintenance and servicing requirements. There is also security of knowledge that the cooling and heating provided by this system comes from your property and will not increase in price.

The actual cost will not only depend on the installation costs but will also have to include the energy demands of the building. For all types of systems the cost will have to comprise the design of the system, the equipment mobilization and the commissioning.

This company installs geothermal heat pumps for Australia and if you check out their FAQ section there are some interesting fact and figures.

Check out http://www.energycore.com.au

Does anyone know of any other company that installs/sells ground source heat pumps?