For cable subscribers, you can subsidize the cable company for around 15 bucks a month for a HD dvr box you don't actually own. That equates to around 540 dollar over a 36 month period. Considering the sub par performance of most cable set top dvrs, that's a boat load of cash for something that isn't quite spacious, reliable nor close to being state-of-the-art.
The value proposition breaks down to how you outlay the money, what you get for your outlay and how a particular platform performs & offers any array of expanding services and capabilities down the road.
The 900 dollar argument isn't easily presented by the pundits, because it is quite frankly not well thought out - or thought through. When you calibrate the discussion of a cable dvr vs Tivo Premiere or Premiere XL the dollar difference investment over the 3 to 4 year period and the value proposition becomes even more compelling.
Tivo works for many.