Over more than five months, the academics used their own funds to execute 85,000 trades in 128 different stocks and made what they consider an important discovery: They were getting significantly different prices to buy and sell shares, depending on which brokerage handled the trade. Extrapolating from the results, they estimate it costs small-time US investors as much as $34 billion a year, said Christopher Schwarz, the finance professor at the University of California, Irvine who wrote the study along with four colleagues.

https://www.bloomberg.com/news/articles/2022-08-22/hidden-price-of-no-fee-trading-34-billion-a-year-study-says?sref=EcccZSTc