It could be that they measured a stack of bills sitting on a table, then did the math to make a metal frame to contain $1,000,000. But they didn't account for stacks compressing under the weight of higher stacks, and it wouldn't look as nice if the top part of the cube was empty.
Still, it does seem like it would be cheaper to rebuild the case than to add $500k to it. Maybe it's easy for the Fed to acquire more cash as long as it's guaranteed not to be spent.
Still, it does seem like it would be cheaper to rebuild the case than to add $500k to it. Maybe it's easy for the Fed to acquire more cash as long as it's guaranteed not to be spent.