Simple cost analysis question
This feels like trying to read your mind, so I’m probably wrong. I will venture an answer regardless :D . I think that what is happening is that the benefit from buying the insurance is certainly the reduction in exposure: A-C. However maybe you are thinking about the remaining exposure after buying the insurance, in that case it is (A-C) + B. So I think you are thinking about the total costs after buying insurance while the question was asking about the change in costs. I prefer the term exposure, but I thought that using your terms will make the exposition  clear. 

P.S. I hope it is understood that we are not literally adding costs, since they occur in different states of the world, but rather using it as a short hand notation to focus on the relevant issues.