Remember that you'll probably be working for at least another 25 years. If your current career is not rewarding to you, you'll probably switch jobs at some point. If you aren't sure what jobs you'll get then, doing an MBA will be a great opportunity for you to explore different paths while building more credentials for your future career. Business school is a concentration of resources - classmates, professors, career center, corporate access, etc. If you want to learn about business and about careers, a top business school is the best place to do that - your learning experience will be rigorous and intensive. Many MBA's go into traditional MBA professions (consulting, finance, marketing, etc.), but many others explore entrepreneurship or craft alternative career paths. Of course the costs are high, but spread over 25 years, they may not amount to much. If you play your cards right, you'll catapult yourself into a rewarding career.
Doing an MBA is a venture; if you aren't sure you want to take the risk and are happy with your status quo, don't do it. If on the other hand you find that you have some tolerance for risk and want to see what else there is in the professional world, an MBA at a top school is worth every penny.