If all you want is a simple dual tuner DVR, you can build one for under $200, and still come out ahead. IF you are not planning on connecting additional TV's, an off-lease PC with windows 7 installed is $100 - $150, and it already has pretty much everything you need to run it as DVR (mainly Windows Media Center). Get a dual CableCARD tuner for $50 - $60, or triple for $99.
Rent CableCARD from TimeWarner for $2.50/month, and you are all set.
You don't need a super duper powerful PC to run as a DVR. A dual core Intel Atom is more powerful than your typical DVR, or even TiVO processor.
6 TV's in the house on FiOS and we only pay $4.99/month to connect them all!!! Power to the CableCard and WMC7!!!