I think when people (I'm guilty of this as well) advertise their "jumps" in MCAT scores, it is quite misleading as the previous poster mentioned. As a teacher for Kaplan, I believe that the course's true measure of value is the "jump" one experiences between the "midterm" full-length MCAT and the actual thing. By the middle of the course, students have already been refreshed on most of the material and have gone through the motions of taking parts of the test. From that point until the real test, the value of Kaplan comes from review sessions, loads of practice tests, and the "hell week(s)" where you are hit with 4 consecutive full-MCAT's (not to mention testing one's 3 D's - drive, desire, and discipline). The question is, can you do this all on your own, making prep courses unnecessary? It all depends on the student. I disagree that all you need is a thick review book. Most books I've seen are either grossly over- or under- packed with the right information. There are these huge books that are basically physics, chem, and bio texts combined into one, probably containing only about 40% information relevant to what will be tested on the MCAT. If nothing else, Kaplan boils down the material to topics that are covered by the MCAT. If you don't want to sign up for the entire course, you can still buy the Kaplan books (although a bit expensive).
Bottom line is this - I don't think that many people are so strapped for cash that $1000 is too high a price to pay for something that can potential have an enormous impact on your career in medicine. If you do a review course and do score much better, that will probably have been the best $1000 you've ever spent...