All bokken will eventually break with continual use.
Cheap bokken are going to break more frequently than better ones, but price alone, is not the sole determinant of whether or not a particular bokken is going to be good for you.
Most cheap bokken are made out of loose-grained, porous red oak, which breaks all too easily. Use some chemical strippers on a cheap bokken, remove the varnish, and you can see where the manufacturers used wood fillers to smooth out the wood surface.
It would certainly be nice to have a lignum vitae bokken, but of course, most of us can't afford one (200+ USD for one, no joking).
Hickory bokken seem to offer a nice blend of strength, resilience, durability, and weight, and they tend to be cheaper than exotic hardwood bokken. Figure on paying about 50-75 bucks for one, but it will last many, many times longer than your red oak bokken will.
Purpleheart, jatoba, etc., are also excellent woods. They're denser and harder than hickory, but they're also more expensive. Figure on paying about 60-100 for such a bokken. These exotic woods are certainly beautiful, though. I still have a jatoba bokken that's been with me for 4 years now. Although it has a few surface cracks, it's still solidly intact, and can still break red oak bokken with relative ease.
Ipe / Brazilian Walnut is an excellent wood for bokken. It's very dense (sinks in water), and doesn't dent easily at all. Much more economical than Lignum Vitae. Figure on paying a bit more for one of these than the above woods, maybe around 70-125.