I recently switched to Comcast and realized they charge $7/month for cable modem. I would suggest buying the Motorola SB6120 SURFboard Broadband Cable Modem from eBay or Amazon which is DOCSIS 3.0 for about $80. You may be able to get for $60 through deals.
With calculations, this means you will own the modem within a year. At present, this is the best cable modem I could find according to the Amazon reviews You may be able to find cheap options but make sure you check the list of approved Comcast Cable Modems.
The Motorola SB6120 cable modem needs a wireless router for WiFi access. So you will also need your own Wireless Router.