In a market like SF, or on Million Dollar Listing, I think the argument towards the commission is that the agent will either save you that much through negotiation (or earn you more through inflation) and/or they will get you something that you couldn't get otherwise (e.g. an unlisted property or a celebrity buyer). That said, I imagine the agents' return-on-arbitrage is actually even higher at the top price points, and even harder to quantify because deal details can be obfuscated (e.g. through LLCs etc).