Unless it's $5/month or more, lease or rent it from the cable company. Why? Two simple reasons.
1) Unless you get very lucky and find a great deal, you're going to spend $50 or more buying this piece of equipment. At $3/month, it would take you 17 months to break even. At 2/month, over two years. at 1/month, over four years.
2) What do you get for that money? A piece of equipment the cable company can blame for your service problems. If it's your modem, it's your problem. If it's their modem, it's their problem.
I've had cable from one company or another for over 6 years. Only once did I own the modem. And that was because the cable company jacked up the rental fee from $0/month (part of a package) to $5/month. And I had a cablemodem left behind by a former roommate. Cable company said the modem he left behind wasn't reported stolen so I switched over to avoid the new charge.
I recently got cablemodem service set up for a relative who wanted to buy a modem. At the time, the best price I could find locally was $70. No rebates anywhere. 70/3 = two years to break even. Didn't make sense. A few weeks later, I found a cablemodem in a thrift store for $10. That made sense. Bought it, switched out the rental, and returned the rental to the cable company. By the end of July, he'll be saving money.