Should you take an expensive prep course? As you might guess, there is no one correct answer.
I do not believe that it is necessary for everyone to take a prep course in order to get the maximum score possible. It is essential, however, that, one way or another, you prepare for the LSAT. With that in mind, there are extensive materials available that cost far less than a traditional prep course:
TheLSAT/LSDAS Registration and Information Book itself contains sample questions covering all three types of questions on the test - reading comprehension, analytical reasoning, and logical reasoning. I recommend that you look over these questions and see if any of the types gives you particular difficulty. If so, and if further work in the materials described below doesn't help, a prep course may be for you.
When you pick up a copy of the Registration and Information Book, you will find in it a copy of a complete sample test. I recommend that you take the test under the same time constraints that you will face the day of the real thing.
Highly recommended: For additional practice, you can purchase various LSAT Preparation kits from Law School Admissions Services. A description of those materials and directions for ordering them appear in the Registration and Information Book, two pages after the Table of Contents (also shown is their website):
For example, for as little as $19, you can buy three past tests; in my view, there is no substitute for just sitting down and working through actual past questions.
For $36, you can order the "Official LSAT PrepKit," which includes three complete practice tests and three interactive workbooks with detailed explanations for both correct and incorrect answers. I've looked at this PrepKit and I think it is extremely valuable.
Check out area bookstores, particularly those serving undergraduate institutions. Several books are available which provide low-cost instruction for the LSAT.
It makes sense to me to skip a full-blown prep course if you are pretty much a self-starter and will practice for the LSAT on your own; if you found the SATs to be fairly straightforward and did well; and if you find the sample LSAT materials also to be reasonably straightforward. On the other hand, a prep course makes sense if you have a history of underperforming on standardized tests; if you are a procrastinator who needs the structure of the course to make sure you will study; if you have looked at the sample LSAT materials and found them mystifying; or if it makes you extremely uncomfortable to know that so many other people (roughly one third of all law school applicants) are taking the prep courses.
One final word to anyone who decides to take a prep course: I don't endorse any particular provider. Remember, however, that you are the consumer with the money to spend. Contact several providers (check the Yellow Pages) and make them explain why you should take their course. And ask if you can change from one instructor to another if you don't like the one you first get.