I believe I rigged up
my air source window unit for under $50. If I had bought all new parts to do the mod, it still would have been under $100. The most expensive part of these projects is the skilled labor involved. I believe geothermal specialists charge even more than a Cadillac dealership for their time and effort. More in line with Ferrari service centers. The sweat equity invested is well worth your time (unless you're a Ferrari mechanic).
There are really no comparable units to what I built. The closest thing is a floorstanding portable unit, which at 15kbtu start at around $500. Next cheaper are the mini-splits...spend however much makes you happy. For a DIY project, I am extremely satisfied with the results.
With a ground source system, the ground loop/loops are arguably the most expensive portion. For a homeowner with some land to work with, a trencher and hdpe water pipe can be gotten from the same store. Just rent a ditch witch that will dig deep enough, then drive along and lay the loop like a water supply line. If feasible conditions exist, the price would be very small compared to drilling boreholes or excavating, both in time and money.