You can't build a "masterpiece" by the current definition for $350/sq ft. If you're talking high end finishes, custom millwork, etc, at least $400/sq ft and probably more.
I believe you are wrong. You can curently build a georgeous house in Watersound, Watercolor, etc with all kinds of beautiful finishes, millwork, tile, etc for about $250 per square foot (and costs continue to fall). A "masterpiece could easily be had for under $350. But, even if you are correct, you could recreate the current $12 million masterpiece
for under $4.5 million.