The 9th ACC games provides ESPN with more valuable content. Plus, if I understand it correctly, it would allow the conference to reopen the current deal. In addition to the money a smart deal would require that a high percentage of the 9th games get carried on ESPN or ESPN 2. By doing that ESPN would have less room for any potential Big East games. And they certainly wouldn't go paying a premium for them. With fewer bidders, the Big East's deal with Versus/NBC Cable or Fox would be less lucrative.
Now the downside of the 9th game is tougher schedules. But the Pac 12 currently plays nine games and the Big Ten is moving that way. I think most ACC schools would be willing to take on the added conference game in exchange for more money. Plus it would help ticket sales and the struggle to find valid and profitable non-conference games.
Look for the 9th game discussion to continue for the next 12 months as the ACC tries to figure out how to play catch up with the other conferences and not get bypassed by the Big East.