I hate how commercial law school admissions has become. That said, I'd say the cost of an LSAT prep course is well worth it in comparison to the potentially large gain you could derive from an improved LSAT score. While Testmasters is a benchmark course, there are other options available (e.g., Kaplan, Velocity, Powerscore, Blueprint). Also, based on the number of practice tests you've taken, it sounds like you have to discipline to consider an online class.