Amazon offers Prime members the Linksys Advanced DOCSIS 3.0 Cable Modem for Comcast for $19.99 with free shipping. That's tied with our mention from two weeks ago as the lowest price we've seen. (It's the best deal now by $15.) It features download speeds up to 340Mb/s, upload speeds up to 120Mb/s, and a Gigabit Ethernet port. It's certified for use with Comcast Xfinity services; check with your provider if you use a different service.
It has generally good reviews on over 1,000 verified purchases at Amazon, although many critical reviews mention a higher-than-usual failure rate after just a few months of use.