Bought a place that had a 30-year-old barn that was in nice shape, but it had a gravel floor. So we took the gravel out down to 8", (and built a nice parking pad with it next to barn/shop). Then insulated beneath and around the slab-to-be, put down heavy-duty vapor barrier, wire reinforcing mesh, then PEX tie-wrapped to the mesh. Place 6" slab on top, with no slope, fiberglass, or expansion joints/cuts. Level and smooth like a warehouse. A little slick when coming in from the snow -- door mats should help. Intent is as a shop with a divided-off office. Separate hydronic zones for each.
Excavation and concrete was $13k for essentially 36x40. I did vapor barrier, placed supplied mesh, and all PEX layout. Concrete was pumped in via trailer pump; 3 trucks. No damage to PEX, based on air pressure readings throughout pour.
Below-slab materials were $1400, above-slab were $3200. All pumps, valves, electric thermostats and controllers, pre-fab sectional system piping, pre-fab PEX manifolds, on-demand direct-vent water heater, and co-axial intake/vent termination included. Not included: gas line (which we had to install on the property to house and shop -- ouch.), long lengths of 3" PVC for intake/exhaust to outside, and miscellaneous copper pipe/fittings to attach above-slab to below-slab subsystems (I'll probably do this in PEX).
Water heater vent system is good for about 70 feet if angled fittings are minimized. Less so if you max the lines out with five fittings each (90 or 45). Water heater is pretty efficient, and has condensate drain, filter, etc. and more computational power than the lunar lander. Above-slab subsystem has temp and pressure gauges, valves for bleed/purge and fill, filters and air purge mechanisms, and three Grundfos pumps to circulate and distribute heated glycol mixture. It's a closed system, so no city water involved.
All install labor aside from concrete/prep and gas line work is me.
I'm several days away from firing it up. We'll see how it goes. When it's running, I can continue to work out there on the insulation (of which there was none.) I had trouble getting motivated to do that when it was 18 deg F out there...
In the summer, the office will get a window air conditioner. With the large doors and windows, I probably won't air condition the shop. We'll see. Barn has a second floor "mezzanine" that's about 24' square, open to the rest of the building. And a 10x40 porch that's currently all outside space. Will probably wall that in for vehicle parking as time permits.