Business-related metric for an optimization problem
On historical data, you can try to determine what would have happened if you add the model before. For each client you proposed some product, calculate the average gain (propensity*cost) of said product and compare with the average gain of said of best product. Say each product cost 100$.
Last year you offered product1 to client1 but he refused. Using your model you know : he only had 20% chances of buying product1 and he had 25% chances of buying product 2. On average you missed out on (25%-20%)*100 = 5 dollars. You can calculate a similar metric on all your clients, than aggregate the total. That would give you an estimation of the amount of dollars you would have made last year if you used your model. There rarely is more business-related metric than money.