If you do the math, $12.95/mo will cost you $466.20 for 3 years. This is much more than a lifetime. Your new Tivo should last at least 3 years, most likely it will last much longer.I also am not interested in paying more then $12.95/mo (lifetime is not an option).
I have a 2003 Series 2 single tuner that's working just fine. I pay $6.95/mo because I also have a Series 3 purchased in 2007 with lifetime, which is still going strong. In 2009 I bought a Tivo HD Series 3 with a lifetime, it had issues and was replaced by Tivo under warranty, but now works great.
My point is, whatever you decide you have to consider the overall cost and not just the monthly fee. Figure out how long you anticipate keeping the device before upgrading and then do the math. Whatever costs you less for that period is what you should do.