This is a discussion on How much does each comcast box cost? within the Comcast forums, part of the Cable and Satellite Providers category.
It depends on where you live. We added a set top box a couple of months ago after getting a TV and adding service in my daughter's room. Cost us $9.99/month. We're now paying $109.99/month
It may also depend on if you have bundled discounts. It's $8 for me.