I'm curious. Is it the case that OTA DVRs like the TiV Roamio, and Tablo, all really make their money on the subscriptions? A $50 machine even with a 500GB drive is making a loss. TiVo makes its money on $14.99 subscriptions. And it sounds like you need one of these subscription because you have no EPG otherwise? And in any case, they won't sell you one without a subscription.
The Freeview system used in the UK, which broadcasts HD and SD channels OTA, has a built in EPG which means that manufacturers can build boxes for one-off payments and no subscriptions, utilising the EPG to give the DVR functionality. You don't get the recommendation engine of TiVo, but you get series link etc.
The key for these things is the EPG, and from what I can gather, every supplier in the US does their own - hence those issues when the football runs long and you don't pad your recordings.
It'd be interesting if someone was to create an open source IP-delivered EPG in the US market, and then see if manufacturers would build DVRs that used it. Add in Netflix/Hulu/Amazon Prime functionality, and you're much closer to being able to cord-cut. It might cost $300-400, but that'd be a true one-off fee.
Adam