According to this analysis https://seekingalpha.com/article/4205379-robinhood-making-mi... HFTs pay Robinhood significantly more for their order flow than other brokerage firms i.e. claiming almost 12x more than E-Trade makes ($260 per $1m in orders vs $22 per $1m). If this is true (I haven't independently verified but these fees must be published publicly by law), then clearly HFTs are getting something more from RH than they are from other brokerages.
The author goes on to say "Robinhood is well on their way to making hundreds of millions of dollars in cash income by selling their customers' orders to the HFT meat grinder. High-frequency traders are not charities. The only reason high-frequency traders would pay Robinhood tens to hundreds of millions of dollars is that they can exploit the retail customers for far more than they pay Robinhood."
The author goes on to say "Robinhood is well on their way to making hundreds of millions of dollars in cash income by selling their customers' orders to the HFT meat grinder. High-frequency traders are not charities. The only reason high-frequency traders would pay Robinhood tens to hundreds of millions of dollars is that they can exploit the retail customers for far more than they pay Robinhood."