They usually go for between $55 and $65 when i see them on e-bay or other places when they have "Boba Fett" in the title or description. However like Andy mentioned, you can get them for much cheaper if you have the time/patience to wait. i found 2 on one divers belt on e-bay once for a mere $25. I sold the weights off the belt for $25. So i essentially got 2 buckles for free. That was a rare occasion but i have found them on the weight belt for between $10 and $30.
If you need it quickly, fullmetalfett does great replicas as well.