I think the main counter argument to it being worth more now is that it was apparently already Banky's most expensive piece to have sold. A less valuable piece

I think the main counter argument to it being worth more now is that it was apparently already Banky's most expensive piece to have sold. A less valuable piece might expect and increase, but it's not clear that that would happen with his most expensive piece. Also worth noting that it went for multiple times the expected price at this auction, which is somewhat suspicious.