You can probably back out what the HFTs margin is on this stuff by looking at public financials... KCG was public for a while.
Essentially, your trades have a lower cost basis than those of institutions. But the markets are generally priced assuming institutional traders. So if firms know you're not an exchange they can save some money on you, and Robinhood literally passes some of those savings to you.
(I don't love Robinhood or anything, but the model makes perfect sense.)
On a more meta level, my firm suspects that Citadel is trying very hard to price other players out of the market even if it's at the cost of their current profitability, so they might be driving up the terms of newly minted deals. I don't work on the retail desk so I don't know.