John,
This is actually one of the best questions ever asked here. My first tendency when I saw this yesterday was to parrot TD's response. Since you mentioned design, I figured it was a new course, not pushing some back tees back.
And yes, while you narrow up at the tee in a real estate course, you have to figure that at 7500 yards or whatever, you still add tens of acres to the design.
You also have to add irrigation back there, grass more, maybe add cart path to get back there, etc. Maintenance, once established, for the first 150 yards can be reduced (if surrounding houses don't object) by planting natives. Typically, the cost of running the irrigation line cannot be reduced. You might not have to run the cart path to the back tee given the 1% who typically play them.
I never thought of building multiple tees as adding tee cost. Based on play levels, you need to build tees of certain size to stand up, whether its one big one or several smaller ones. I have taken to building back tees (except par 3's) about 15 x 15 feet and even those don't see their share of action in most cases.
Statistically, fw and corridor width really aren't that much different for all players. The "extra" cost of those comes from going up a row in sprinklers and/or not matching to them well, necessitating even more of them.
While we try to build courses for all people, the length that better players now hit it has stretched the amount of golf course needed from maybe 400 yards to 6-800 yards extra, specifically for them.