The Emerald Club is $50, but I think until the end of August you can get it free with a Disney Club membership. You can call National, they can fax you the form with "fee waived" on it, and then fax or mail it back, or take it directly to a National counter.
For exact deadline, check the Disney Club website. (It would be worth the DC membership if you don't already have it.)
If you have anymore questions, you might want to check the transportation board; although much of the specific info is WDW related, you can get other general info about National, the Emerald Club, the Emerald Aisle, and getting the membership for free.
Good luck!