My score too went up 10 points from the diagnostic - and in my opinion the Kaplan course was *absolutely* worth the money. No way I would have done as well as I did with out it.
I'm somebody who procrastinates, and there's so much to study, I loved the structure of the classes and helping point out exactly what you need to know content-wise for the test. The timing of the test and sections is so insanely crazy (although I don't know how it will change once the computer-based test is implemented), that the most important things Kaplan gives you are the best test-taking strategies, and the full length practice tests. By the time you take the actual test, you'll have done at least 5 full length tests, which are structured exactly like the actual test, so you'll be used to the length and sheer fatigue it wreaks on you. And also, the kaplan tests are often much harder than the real thing - I did a lot of the old AMCAS released tests on my own too - but they're great practice.
And as far as the money goes, yes its expensive. But put it into perspective - its allowing you (hopefully) a better shot at getting into medical school, and eventually, once you are established, that money isn't going to matter to you. It's paying in a little bit now to get tons out later.
And one last thing: Always keep in mind that its in Kaplan's best interest, as a business, for you to do as well as possible. That's how they make their money - and lord knows they want to make money!