The bull market is on sale for online brokerage customers.

In the run-up to  22nd annual ranking of  Best Online Brokers, two developments topped all others: The three major indexes—the Dow Jones Industrial Average, the Standard & Poor’s 500, and the Nasdaq Composite—all set record highs, and six brokers slashed commissions and other fees. At the 16 brokers we reviewed this year, the average monthly cost of trading for an occasional investor had fallen 25% since early 2016, to $47.51 from $63.45. That means more of the market’s gains are ending up in the pockets of self-directed online traders.