Depends on your style of practice. Some styles like to "clack" bokken together, consequently a low grade inexpensive bokken will probably break. However, when I first started training, we did a lot of such "clacking", but from the stand point of deflecting, not blocking, consequently even the cheap bokken lasted a long time.
I would not recommend buying an expensive bokken if you are going to be going up against someone swinging one of Bujin's laminated bokken.
In my store, I carry a line of hand made bokken made from purple heart wood, ash, cocobolo, bubinga, chechen and yellow heart. I even have a couple which are laminated and will stand up to anything out there. They all vary in density and would probably stand up to a good beating, but not if you are going to be hitting bricks.
Best is to ask your sensei what he/she would prefer you to start with.