(I'm not in Comcast area and I only use OTA... take this with a grain of salt.)
From what I've read on tivo forums, if you call Tivo directly, they'll make you a highly discounted deal on their current offerings with lifetime subscription. Note that I didn't say "a good deal." I said "highly discounted deal." Whether you want to pursue that... up to you.
Your lifetime subscription box can then be sold on ebay. Act now, before more cable companies (or more Comcast markets) switch to mpeg4. I am guessing we are talking a series 2 or series 3 box here. I think Romios and Premeires DO upgrade themselves automatically to handle mpeg4.
Historically, I've been a huge Tivo fan... but ... their pricing has just gotten nuts. I suspect when mine dies, I'll either fully embrace watching things online (or a year later on dvd) or I will fall back to building my own MythTV type box. In the old days, it was much cheaper to buy a lifetime Tivo than to buy the equivalent hardware to build your own. Now: I haven't done the math, but I'm pretty sure we crossed that threshold a few years back.