Trading costs can be divided into explicit costs and implicit costs. The explicit cost is the commission charged by the stock broker. The implicit costs are the price impact of the trade, and the opportunity costs when a limit order is not filled (e.g. due to delays or bad order routing). The implicit costs are difficult for retail investors to measure, although I would assume that the price impact of a small trade is negligible for liquid stocks.
The explicit transaction cost is zero when using a commission-free stock brokerage, but what about the implicit costs? Is there any evidence to suggest that zero commission stock brokers are cheaper overall for retail investors, when compared to those stock brokers that charge commissions?