they live in US and they arent building them to hold snow, lot of them build in 2x4.. very little insulation, slab of 5-6" without surrounding wall, etc... its easy to throw low number also.. (like the HP in a car..)
i would also point out that the material seem less expensive the other side of the borders.. over here each m³ of concrete is around 140$ i think in the US its like 75$...
here in Canada, 30x40, no less than 25k$ and im easy, i would say 30k$ if you count a real outside and maybe a carpot.
the rough isnt expensive but still all 2x6x12 + 7/16 press wood + forrences (count for 15% of your build normally), its all the other thing like roofing, nails and screw, insulation (R20 in wall and R40 in ceiling, Enermax...) + pink foam to insulate the slab from the exterior (perimeter)... some windows with double glass, 1-2 garage door at 1k$ each, propane/natural gas heater, exterior finition and i forget lots of stuff.
im into the process of building a 30x40, i live in Canada, Quebec, its not cheap if you dont want to cheap out too much. to each his own...
sorry for my english, its not my first language