I'd like to recommend the book "trading at the speed of light". It's an in depth look at the world of high frequency trading based on first hand interviews with traders, court documents, etc. It would appear based on what little information is actually available that HFT firms net only modest profits after taxes and fees on trades. Historically, it appears most firms fail to turn a profit and shut down. A paper of Laughlin (2014) suggests virtu, a hft firm that now executes Robinhood orders, earns an estimate profit of 0.24 cents per trade, with 0.05 to 0.1 cents per share traded being a respectable profit.
I highly recommend this book though if your into this topic