I have both receivers in the living room and my wife and kids still use the TiVo far more than the HR21 since the UI is easier to use and most of their programming is SD. I also record network programs off the air in HD off the TiVo as a backup in case something goes wrong; there have been times when the TiVo hiccuped and I have it on the HR21, or vice versa.
If you enable the TiVo with DirecTV it can hold 200 hours of SD programming and/or record HD programs off the air (OTA). (
Good antenna needed, go to
www.antennaweb.org for advice.) I'm in LA so I get a lot of HD OTA content.
I don't think DTV will charge more in DVR or HD fees for hooking up a third DVR, but you should check to be 100% certain.
You will of course need another set of cables to go from the sat dish to the Tivo. Depending on your current hardware you may also need a new switcher to accommodate the third pair of coax cables. I have a rig like this with the new Slimline dish and the HD TiVo works just fine. Bear in mind there's no satellite-based HD programming left for the TiVo, though; it's all on the HR21 now.
If you're really keen on doing this and don't feel like running cable, swapping out switchers, climbing on the roof, etc. etc. the $80 may be worth it.
Hope that helps...