Ben, I'm not sure all the costs will be halved.
You have to build a maintenance building and a clubhouse. Could you cut them in half for a nine hole course? Doubtful. Even if you did there are fixed costs in any construction project that couldn't be halved. A half size clubhouse will still need a kitchen, locker room, bar (I hope!) that would reflect the size of the membership. Could have the crew maintain a nine hoke course? You would need one superintendent, one GM, one pro, although I love the mom-and-pop model where some of these roles could be combined.
Don't get me wrong, I'm a big fan of nine hole courses. I played quite a bit as a kid at the Mare Island course in Vallejo, Calif, and really enjoyed it with completely different back nine tees. I just don't think it's realistic to think costs would be cut in half for a nine holer.