Anything related to GMAT is an expensive affair. GMAT preparation is not only a time investment but also a huge monetary investment, if you decide to join a test prep company or private tutor.
There can be various reasons to join test prep or hire private tutor. One must keep few factors in mind before taking the plunge.
1. Do you really need prep course - Its not easy to open book after X years and commit yourself for Y hours daily. Or it can be lack of commitment due to work. Or the first time you tried yourself did not get the result you wanted. Whatever the reason, once you are convinced that you need prep help - do your homework.
2. In Person or Online - Many companies offer introductory sessions which are absolutely essential to get feel of their approach to GMAT and whether it will fit your study style and more importantly will be able to help you to get the desired results. These sessions also provide an opportunity to ask follow up questions from your perspective tutor which will facilitate your decision making.
3. Track Record of Company/Tutor - Go beyond what the testimonials say about them. Do your homework. Ask for feedback from your friends about the company/tutor you are inclined to join. Ask for their experience and what they felt about their coaching style, support etc. etc.
4. Talk to your perspective tutor - Get to know his/her teaching style. Discuss about your weak areas and how he may be able to help. Most importantly, ask when was the last time he gave GMAT. Mostly all tutors are 99 percentilers but the important question is when was the last time they appeared for GMAT. This in my view is the most important question one should ask before joining any preparation course.
5. In Class/Private Tutoring - Again it depends on what areas you need to concentrate, time schedule and how much you can spend on tutoring. If you need focus only in specific area, say verbal, go for private tutoring. However, if you want consistancy in your prep and thats the reason for taking a class, go for in class prep. Other factors such as how soon you want to give GMAT, how much time you can devote for classes etc will swing your decision.
There will be other factors too but these are in my view top 5 factors one must keep in mind before deciding on GMAT prep course.