Many study course providers claim they can produce astronomical gains in your SAT results. These claims, unfortunately, do not tend to be backed up with hard, reliable data. This is not to say that SAT prep courses are worthless, but they are not generally as valuable as the providers tend to claim; rather they are somewhere in between.
SAT courses are not cheap -- they tend to cost over $1,000 per course, and in the hundreds of dollars per hour for one-on-one tutoring. This is another factor that you should look at when thinking about an SAT course; look at it as an investment. Regardless of the results you get, is it worth $1,000? Do you stand to get $1,000 of value from your boosted SAT scores? Again, while SAT courses do have some benefits, they may not be helpful enough to be worth this kind of money, although this decision is ultimately dependent on your goals, values and finances.
While an SAT prep course may be helpful in terms of points on your SAT, it may not be helpful in terms of actual skills. According to a 2005 Washington Post story, for example, some of the tips were to write the essay in large letters and use multi-syllabic words whether they are necessary or not. These skills are helpful for getting higher SAT scores, but there are not likely to be many situations in real life where large writing saves the day.
In spite of the above, SAT prep courses do have a track record of increasing average scores; however, they do not tend to increase by much. What's more, the comparison is often made between the actual SAT and the prep courses' mock exams given to students, which many have derided as being unfairly graded or harder than the actual test.
Having said this, the testing-taking skills and general drilling do increase average test scores. Just don't expect any miracles.