GameStop says that attach rates of DLC to games have gone up, as it is easier to sell than other game products. The DLC for Gears of War 3 was heavily promoted and got a 10 percent attach rate (better than the two percent for the first two games) and a good turn around considering 75 percent of GameStop’s Power Up Rewards members said they weren’t aware of the DLC before the game’s launch.
“Downloadable content is easier for us to sell compared to strategy guides or accessories,” GameStop’s director of retail digital distribution, Brad Schleisser said. “They’re games and our sales associates are gamers.”

Attach rates have increased steadily over the past year. For instance, 17 percent of Call of Duty: Black Ops 2 players paid GameStop for Elite and 30 percent paid for Mass Effect 3‘s From Ashes.
Source: Destructoid