The price of traffic in a RTB environment is very dynamic. In a real-time auction, where practically every impression is sold for a different price, the answer would be another question: “The price for which impression?”
There are many factors that play into the price of each impression, but ultimately prices are dictated by supply and demand. For example, if nobody wants to buy Brazilian traffic from msnbc.com, then the cost might effectively work out to $0.25 CPM (hypothetically). If everybody wants to buy US traffic on cnn.com during Christmas season, the effective price of traffic might end up being $10+ CPM (hypothetically).
As the name implies, RTB operates on an auction model. You set the max bid (CPM) that you will pay for a placement and win impressions at $0.01 above the next highest bidder.