The price of traffic in a real-time bidding environment is very dynamic. Every impression is sold for a unique price, depending on the market conditions at that moment. The answer to "how much does inventory cost," would be another question: “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 the Christmas season, the effective price of traffic might end up being $10+ CPM (hypothetically).