I think you are comparing apples to oranges. Price 12-2 w/ ground.
If are you going to dig a shallow trenc by hand, conduit will be better.
I would use 3/4 inch conduit and run 4 wires, 220v. A 220v heater will use 1/2 the current so 12g wouldn't be a problem.
Just do it and see if you like it. I too would use what I have before spending money.
If this wiring is going to be used for a few weeks a year, I would use extension cords, have over 450 ft of 12g cords.