Cheapest place to buy insulation

I'm converting my detached 20'x23' garage into a studio apartment (with permits for everything). It was built in the 1930's. My contractor is suggesting that we put a 6mil vapor barrier on the old 4" concrete slab, followed by 2x6's on-edge, then fill the cavities with R-21 kraft covered insulation batts and then plywood subfloor over that. The local city inspector says this will be fine. Also, the footprint of the building is being extended about 2 feet beyond the old concrete slab (in other words, the wall where the original garage door was is going to be 2' further out). He has poured a new foundation wall at that point but he wants to just leave the interior floor area with dirt and put 6 mil plastic over that section before installing his floor joists. Does all this sound correct? I'm concerned that moisture will migrate up from the concrete (and dirt in that one area) and stay inside the floor joists, getting the batts wet over time.

I have purchased an 8 X 10 wooden shed. It has only the 2 front doors. No windows. I plan to wire it for lighting, air conditioning, and heat. I was thinking of insulating the walls with batt insulation and covering that with osb. I was thinking I could make a ceiling out of the pink sheet insulation perhaps 1/2 inch thick and seal all the seams. I only need to keep this shed at a temperature of about 60 degrees year round, hence the heat and air conditioning. I know the walls will be fine but do you think I will need more ceiling insulation? Also since it is a portable building that is built on skids how do I insulate the floor or will I need to do the floor? I was thinking of putting a coating on the floor (plywood) that could be washed easily. I live in Southern Illinois so we have mostly moderate temps with a few extremes per year. Does this sound like a firm plan?