Check around in your area, don't take the first estimate as the real number. There can be a lot of variation from contractor to contractor. However, with geothermal, there might not be too many choices.
We looked into it maybe10-12 years ago when we were looking into building because we couldn't find land with a small house that we could afford (found one since). If I remember correctly, it would be 12,000.....or was it 20? I think it was less if building from the ground up, more for an existing house. There was only one company around here at the time.
I do remember that the systems were known to rarely need any repair and not much maintainence. And with the savings, the entire cost would be recouped in 10 years with average use, and they system would last at least 50, they weren't old enough to know how much longer they would last. So very cost effective in the long run.
I would definitely look into it if we were to build in the future, or if the town ever gets to our house with city sewage, so we wouldn't have the leach field right behind the house. They have to dig a hole the size of a swimming pool near the house for the system to be underground. We don't have that here with the leaching field. Our lot is too narrow to do anything on either side or the front.