Don't focus on the cheapest - get a good one. IMHO Motorola makes the very best cable modems - look for a Motorola Surfboard SB61xx. The SB6121 is a great choice & you can get it for $78 from Newegg: http://www.newegg.com/Product/Pro...6825122015
If that's more than you wanted to spend, go pick up a DOCSIS 2.0 from Craigslist, which would be the Surfboard SB51xx. Unless you're paying for the Extreme 105 ($199/mo) from Comcast, then you don't NEED a DOCSIS 3.0 - a 2.0 will work just fine, and you can snag one of those for about $25 used.