Walmart offers the refurbished Arris SurfBoard DOCSIS 3.0 Cable Modem, model no. SB6141, for $26. Opt for in-store pickup to avoid the $5.99 shipping charge. That's $4 under the lowest price we could find for a refurb elsewhere, although we saw a used unit for $3 less three weeks ago. It features support for IPv4 and IPv6 and enhanced network management. Note that it may not be compatible with all service providers.
A 90-day warranty applies, but it's unclear who backs it.