I'm looking into it too.
From what I can tell, it's about $7 a month or so (Comcast). I'm guess they'll also charge an activation/deactivation fee.
The only concern I have, is that I work until 3:30 pm every weekend, so I'm not sure I'd get my money's worth, only using it for later games. If I had Sundays off, I'd definitely buy it.