Plastic is a moisture barrier.  Warm floor pulls moisture and robs heat? true or not not sure, Just use a thin cheap silage tarp.

Sand?  You would be heating the sand first then the concrete.  Too me I want to heat the concrete, which is the original goal in the first place, so put the tube in the concrete. 

Price not sure where you are and how much tubeing they are putting in, so doesn't sound real out of line.

I had to put a power vent on the LP gas boiler in the one shop.  would not vent right without it?  My shop at my house I used a tankless lp water heater on 1200 square feet,  like it LOTS better than the 40 gal I started with.  If the boiler in the shopin town goes down, think one ofthe bigger tankless will work, it is 2400 square feet and I would have to look at the papers again on the btu's to make sure.  For some reason I just don't like that little lp boiler i got!?  The other building has a monster clean burn oil boiler in it and that seems to work really good with lots of cleaning though.

JMHO