Apart from the money they take from people directly, and the expenses of going to meetings and assemblies, buying suits etc... (for many, it's the only time they need to wear one), the biggest cost is time.
You can do so much with your time once you leave, it's really THE most valuable commodity anyone has and why people work - to purchase future time not working.
Away from the WTS you can develop your career, start and run a business, educate yourself, or heck, just sit and relax for a while, spend time cultivating real meaningful friendships.