It's obviously hard for me to decide what's right for you. Unlike advice columnists, I don't presume to know what fits your situation best based on a short description. But your concern is both apparent and fairly universal, so it should be addressed.

Courses for the GRE, GMAT, LSAT and other standardized tests are expensive. That's because the testing companies have a lot to pay for. Classrooms, support personnel, marketing, overhead, and basic profit motives require healthy margins. But none of those things make you better at the test. You benefit primarily from two things - great instructors and useful materials - which together represent perhaps 20-30% of the $1200 average student fee.

I point out the math because it shows where your money is going. If you could spend that yourself, you'd probably put it all towards instruction. But then again, you wouldn't be in business.

Should you take a class? Only if the marginal benefits of a real-life instructor, personal feedback, and regimented study are worth the additional $400 for an online course or an additional $900 for a classroom. Otherwise, get the books and study on your own. It'll be a more efficient use of your money, and may provide similar benefits.

One final note: you should be realistic about the prospects for improvement. If you did horribly on the SAT, you are not going to ace the GRE. If you are scoring around 900 in practice tests without studying, you might be able to raise that 100-200 points with assiduous study.

Will 100 points get you into the school you want, and launch you on the career of your dreams? Then it'd be worth it. But don't forget the other parts of the application process, because those are going to be equally (if not more) important to the same goal.