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Value of a call option spread
I think you're missing the fact that the trader bought the $40 call but wrote the $45 call -- i.e. someone else bought the $45 call from him.  That's why you have to subtract 600-100.  At expiration, the following happens: So $600 + -$100 = $500 total profit. Note: In reality he would probably use the shares he gets from the first call to satisfy the shares he owes on the second call, so the math is even simpler: