How expensive would a solid piece of metal that size be?
For just that weight of metal...not too expensive. To get it formed into that shape, potentially very expensive. It looks like aluminum. Someone posted 23.5" x 23.5" x ~144" as the dimensions. This comes out to be 46 cubic feet, and would weigh about 7,700 lbs. A floor of the price would be ~40 cents/lb scrap price, or ~$3,100. Ballpark 80-120 cents/lb might be a more realistic price if it were straightforward to man…
It seems unlikely that there are serious tolerances to be kept here, couldn't this just be cast messily using a sand/clay mould and then cleaned up with portable power tools?
That way you don't need any special equipment and you can use whatever Aluminium you can get your hands on. Hell, you can even recycle Aluminium cans.