Newegg offers the refurbished Zoom DOCSIS 3.0 Cable Modem, model no. 5345, for $24.99 with free shipping. That's $20 under the lowest total price we could find for a new unit. It features support for IPv4 and IPv6 and enhanced network management. Deal ends January 23.
A 90-day warranty applies, although it's unclear who backs it.
There's a less expensive way, and then there's the way that works for you. The two may not be the same.