When you add all the support costs, fuel, tanking, manning, logistics and all the enablers required to bring 2 Tornados to bear the price of a DMB is tiny.
If you add all the support costs, fuel, tanking, manning, logistics and all the enablers required to bring 2 Tornados to bear, yet fire or drop nothing because the cheaper weapon you have does not meet the RoE / CDE, you end up with quite a wasteful and fruitless expense on your hands.
Having 3 types of weapons on-board, with a brace of different fusing and targeting options plus a guy in the capacity seat operating a good sensor is a cost-effective capability.