We have insulation under our basement slab, but it was put in place during the build (still in progress). We're building a full ICF house and the basement floor layers, from bottom to top, consist of pea gravel, 2" EPS foam sheets, plastic sheet, thin rebar grid and a 3" concrete slab. Sorry I can't supply the results or cost since it's all part of the build, but the builder said his other homes maintain low 70's F in the basement with this setup.

Thanks. Do you think the purpose of the rebar is to hold everything in place? I dont think they use it in basement floors around here - but I could be wrong.

Yeah, it's a very thin grid so it's used to mitigate cracking. I think it's pretty standard in slab pours.