ETF prices fluctuate according to supply and demand, which isn�t likely to affect your costs significantly if you�re trading a fund that tracks a major index. More specialized ETFs that trade less frequently may have a wider �bid-ask spread� (the difference between the price sellers want and what buyers are willing to pay). That spread will be evident when you place an order. You could end up buying at a relatively high cost or selling at a low one, which could add to your costs.