Various threads have discussed the probability and value of having an empty middle seat next to you when flying in Economy. Of course, you can guarantee that the seat next to you will be empty by buying two tickets, but that's usually way too expensive. Still, after spending a couple of six-hour flights in Economy--one with the middle seat empty, the other with it filled--I wonder whether there's a less-expensive option UA could offer.

If I've paid $300 for a seat in E+ (one segment), I'm not going to pay another $300 to keep the seat next to me empty, but I'd probably be willing to pay more than, say, someone in E- would pay to take that seat. On the other hand, I don't want to commit to paying for an empty seat in E if I have a chance of being upgraded to F.

What do others think of this? Is there interest in such a product? What do you think it's worth, in dollars or miles? Is there a feasible way to implement it? Are other carriers already doing this?