Cursory search suggests prices ranging from $400 to $1,800,000 per round for what I assume are wildly different types of 155mm ammunition, though I'm willing to bet the $1,800,000 figure is the total cost of a canceled production program that planned to deliver a large number of rounds, amortized across the small number of rounds actually delivered before the program was canceled.

At $1,800,000/round, firing hollow rounds loosely packed with $100 bills and a note promising another volley as soon as the target agrees to stand down would probably be a more cost-effective option.