A lot varies on lot size, house size and geography but we can give you a general idea of what to expect.  The units themselves run at about a 50-100% premium to traditional HVACs.  However, on top of that you need to plan to spend anywhere from $1000 to $3000 per ton of capacity to install your ground loop.  If you can go horizontal or use a pond, you can keep costs on the lower side of that range.  However, if you live in rocky terrain or an extreme climate you will likely need a vertical loop which tends to be more costly.  For a standard house, you can estimate 1 ton of capacity required for every 500 square feet and figure your costs from there. 

As you can see, retrofits can be pretty costly.  We generally recommend our clients install geothermal in new construction projects so that they get a more instantaneous payback by rolling the cost into their mortgage.  That way while the mortgage goes up, totally monthly expenditures do not since the increase is offset by lower utilities. 

Hope that helps give you a better idea what to expect.