Say you're an online publisher thinking about investing in your own, private ad exchange. How big do you need to be to potentially profit from such an investment? Measured in monthly pageviews, no smaller than 2 million, give or take, says Christine Bensen, National Lead for Digital Media at iCrossing. And, even at that scale, publishers may want to think twice about building out an exchange, according to Bensen. If publishers aren't desperate for the money, she told OMMA RTB attendees on Thursday, they should weigh potential profits with the degree to which they're probably going to "annoy" their audience by retargeting ads.