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Why is the bid-ask spread considered a cost?
As an aside, on most securities with a spread of the minimum tick, there would be no bid ask spread if so-called "locked markets", where the price of the best bid on one exchange is equal to the price of the best ask on another, were permitted. It is currently forbidden for a security to have posted orders having the same price for both bid and ask even though they're on different exchanges. Option spreads would narrow as well as a result.