weight verses use
One way to determine if the bokken you have is right for practice is to practice ... say 1000 cuts. This will determine right away if it is too heavy or just right.
Another way to cheaply see if you need a heavier piece of wood, is to get a heavier piece of wood, either an ax handle, or four foot replacement handle from your local hardware store, or shovel supply store.
It isn't as pretty as a finished bokken, but for home practice, and strengthening your arms and wrists it works out pretty good.
I have two about the house bokkens that are fashioned from ten dollar replacement handles that serve very well for practice purposes.
The weight of the practice wood is about the same if not a bit more than the heavier commercial bokkens that range from $80 to $120. Most of the time, during class, it makes no difference as to using either the cheapest made bokken or the best as there is no direct contact made, only glancing contact, if that.
Before you discard the lighter bokken, try the ax handle, sledge handle trick first with 1000 cuts, it will either be too hard or tell you you can go to heavier wood.
There are some pretty good articles on types of wood to make Bokkens on the Aikido Faq ... at least there were, as things keep changing.