Peter,
It would be nice if, say, Jeff Brauer chimed in and explained what the cost of adding yardage actually is.
But I can tell you one cost - building a 7,200/7,500 yard course does little to nothing toward promoting the game of golf, and the side effect of such developments is that they prop up all the other silliness that surrounds the business and the game. It's a marketing tool that leads to an upward spiral in costs and a downward spiral in participation, and it makes it that much harder to build a thoughtful course at a yardage that accommodates 90% of all non-tour players. We have enough courses measuring over 7,000 yards (like 2/3s of the top 100) to satisfy the needs of the long players, it's time to change the focus.