More likely, it's a mutual mistake - both parties agreed to something and then mistakenly wrote it down incorrectly. But even that doesn't seem right to me.

What it feels like to me is a basic question of resolving an ambiguity in a contract. Your jurisdiction will have case law spelling out a methodology for resolving ambiguities. Assuming that the contract states the equation you mentioned (A+B-C = the payment price) and then just does the math wrong, which is what it sounds like from your comments, then it's a pretty simple question of, "what did these parties intend when they wrote this thing?"

My opinion, if I were a judge reviewing or a lawyer representing either party, is that the contract is actually pretty clear - both parties intended the correct amount, the calculation makes it clear what that amount is supposed to be. The ambiguity from the miscalculation would probably make parol evidence admissible. With all of that, I would advise a client to pay the intended amount.