Amazon has the 3Com 3CR29210 Cable Modem for $85.49 with a $20 rebate on top of it. A couple weeks ago, it was at $89.99.
I got one of these to avoid rental fees from the cable company. It works nicely on ATTBI's system here in the NW suburbs of Chicago, but there have been some stories of the modem's incompatibility with some systems.
3Com no longer manufactures this modem, and I've heard that support is almost non-existent, so caveat emptor.
That said :-), here's the modem:
and for those of you on ATTBI, here's a link on DSLReports that tells you how to provision the modem: