First–full-disclosure–I teach a GRE prep course and an SAT prep course for Texas A&M University.
Expensive courses can be worth it if you actually get the increases they claim. They could easily make you ten times as much as they cost, in the form of scholarships, fellowships, or better jobs! 
What little independent research there is on the effectiveness of such courses shows little or no increase in score for those who buy the prep books (although buying the books is not the same as reading the books). Those who take prep classes show some improvement, and the greatest increases are among those who get personal tutoring. This research was specifically on the SAT, but the two tests are very similar. On the other hand, GRE students tend to be more self-motivated students than SAT students, so they might get better results from the books.
Pricier doesn’t mean better. Most courses teach pretty much the same stuff, because they all read each others books and integrate any new techniques they find. For the price of a classroom course from Kaplan or Princeton you can get live, one-on-one, GRE tutoring via the internet. That link is for my tutoring, but I’m sure there are lots more tutors out there. (more…)
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