I think the Kaplan course can be a valuable tool for people who a) don't have the discipline to force themselves to sit down with several books and really study for the exam or b) have tried self-study, but need someone to explain the various concepts in greater detail than is given in books.
If you can buy study books (I used Princeton Review, Kaplan Premier and the Official Guide) and really sit down and devote yourself to spending an hour or two a night to learning - and practicing - the concepts and strategies in the books, save yourself the $1300. If you need the structure of a teacher, a classroom and homework, you'll want to make that investment.
Fortunately, for you, there is time. You can try both methods if you want: take a full-length practice test now, study study study for 3-4 weeks and then take another practice test. If you don't show significant improvement, take Kaplan's 1-month program.