You could always go the lifetime service route. It adds $400 to the upfront cost ($300 for any subsequent TiVo).
With a $20/month DVR rental fee and $5/month CableCard rental fee you'd break even in less than four years. Even sooner if you bought multiple TiVos with lifetime service plans.
I would estimate that you can reasonably expect a TiVo to last about 5 years, so in the long run it's cheaper to buy TiVo and you get a much better experience.