Newegg via eBay offers the refurbished Motorola Arris Surfboard 600 Series DOCSIS 3.0 Broadband Cable Modem for the in-cart price of $39.99 with free shipping. That's $10 under our February refurb mention and the lowest price we've seen. (It's the best we could find for a refurb today by $9, and $40 under the lowest we could find for a new one.) It features download speeds of more than 600Mbps and upload speeds of more than 100Mbps.
A 90-day warranty applies, but it's unclear who backs it.