Hello Joe, you may be right when it comes to some rare materials, usual oak or birch on the other hand are cheap. I'll have to find out more about the prices.
More than the material costs, it's a question of what your time is worth. For most of us, the time spent crafting a bokken would be worth a lot more than the money saved. If, on the other hand, you're a skilled woodworker, or you just want to do it as a hobby, that's a very different matter. But bokken aren't that expensive, particularly considering how long they typically last, so for most of us, saving money wouldn't be a reason to make our own bokken.