A Securities and Exchange Commission experiment designed to stimulate trading in shares of smaller companies has cost investors more than $300m in the past two years, according to a study released Thursday.

The study by brokerage Pragma Securities adds to a growing body of research that suggests regulators’ Tick Size Pilot Program has added to investors’ costs without doing much for the small and midsize companies it was meant to help. The program was mandated by the SEC after pressure from some members of Congress.