I bought my own modem that is just a DOCSIS 3.0 modem without a wi-fi router. No "rental" fee.
List of approved modems.
http://mydeviceinfo.comcast.net/

I bought my DOCSIS 3.0 with telephony modem from ebay for about $67 since I have my phone through them too. Modems without telephony are much cheaper and easier to find.