Amazon offers Prime members the Linksys Advanced DOCSIS 3.0 Cable Modem, model no. DPC3008-CC, for $19.99 with free shipping. (Non-members can spend $49 to qualify for free shipping; otherwise, shipping costs $8.74.) That's a $25 drop since our April mention and the lowest total price we've seen. (It's the best price we could find now by $38.) It supports download speeds of up to 340Mbps. It's certified for use with Comcast Xfinity services; check with your provider if you use a different service.
Update: Note that this item is temporarily out of stock, but can still be ordered at this price for delivery when available.