Some data points from an old article in iQ Magazine: "Advertising's New Game", Michael Boland, Nov-Dec, 2003:
"The price of in-game product placements starts at about $50,000 and can exceed $500,000, which is small compared to the millions that companies spend for product placement in major movies."
"Revenue from in-game advertising for PC and console games (including product placement) will reach $975 million in 2005, according to Forrester Research estimates. Michael Oxman agrees. "It's an efficient media buy," he says, adding that the cost per 1,000 impressions is about $1.50 to $5; the comparative cost for television advertising is about $20 to $25."
Michael Oxman was "managing partner at JAM International Partners, which helps game publishers find ad partners."