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Why is the bid-ask spread considered a cost?
This is a misconception. One of the explanations is that if you buy at the ask price and want   to sell it right away, you can only sell at the bid price. This is incorrect. There are no two separate bid and ask prices. The price you buy (your "bid") is the same price someone else sells (their "sell"). The same goes when you sell - the price you sell at is the price someone else buys. There's no spread with stocks. Emphasized it on purpose, because many people (especially those who gamble on stock exchange without knowing what they're doing) don't understand how the stock market works. On the stock exchange, the transaction price is the match between the bid price and the ask price. Thus, on any given transaction, bid always equals ask. There's no spread. There is spread with commodities (if you buy it directly, especially), contracts, mutual funds and other kinds of brokered transactions that go through a third party. The difference (spread) is that third party's fee for assuming part of the risk in the transaction, and is indeed added to your cost (indirectly, in the way you described). These transactions don't go directly between a seller and a buyer. For example, there's no buyer when you redeem some of your mutual fund - the fund pays you money. So the fund assumes certain risk, which is why there's a spread in the prices to invest and to redeem. Similarly with commodities: when you buy a gold bar - you buy it from a dealer, who needs to keep a stock. Thus, the dealer will not buy from you at the same price: there's a premium on sale and a discount on buy, which is a spread, to compensate the dealer for the risk of keeping a stock.