There is a lot of discussion in this forum about XPS on the walls but not as much about it on the floor. How common is it to insulate the floor with XPS? How much is the 1/2" plywood in your area?
Even though I'm in a cold zone, it's extremely uncommon to insulate the floor simply because it's not Code required. I expect the payback would be quite long, far exceeding the original homeowner's occupancy. Below-grade wall insulation is needed for mould prevention, but that does not apply to the floors. However, certain types of flooring materials would work better with foam underlayments.
I used 1/2" XPS under 1/2" plywood on the floor of our now finished walk-out basement. It seemed to help the temperature of the basement, but I can't seperate how much impact insulating the floor had versus the walls and rim joist area. Installation was difficult to say the least, since I preferred using screws for fasteners. We used carpet, wood, and a small amount of tile flooring in the basement, so at least the installation of those was much easier. The floor isn't hard enough now to be a problem for my kids, which was the main goal.