Bid/ask spreads

The hidden per-trade cost: the gap between the best buying and selling price.

The bid is the highest price buyers are showing; the ask is the lowest price sellers are showing. The spread between them is a real transaction cost: buy at the ask, sell at the bid, and you have paid the spread. For the largest ETFs it can be a single cent; for small or exotic funds it can be several tenths of a percent.

Spreads widen when the underlying assets are volatile or hard to price, when the fund is small, and at the edges of the trading day. They are a bigger deal for frequent traders than for long-term holders, but everyone pays them.

Use limit orders to control your execution price, and compare a fund's typical spread with its expense ratio: for short holding periods, trading costs can dwarf the annual fee.