Funny, I find measuring opportunity costs much much harder than measuring engineering effort.

Even in your example, are those phone calls the substantial costs, or is it customer confidence in your product being lost the main motivation you should do something with the bug? If so, what's the dollar value of this confidence? Also, do you have monitoring good enough in place to see how many customers are hitting the problem every day without telling you? Do you think these customers are less or more frustrated than the one they are calling you?

Can you attribute lost customer to a particular feature not present, to a particular bug the customer experienced just before they refused to deal with your software anymore?