It would be awesome if it was included in with the usual PlayStation Plus sub, but if not I would feel comfortable paying about the same as Netflix. Anything over $10 a month and I would pry end up passing on it.
I think they'll be fine, like you said while the Wii U isn't selling awesomely it is not gonna be a company destroying bust. Also if I'm not mistaken since they dropped the price of the 3DS it's been doing very well.
I will be very interested to see how they will present this if this does actually turn out to be the truth. I can't imagine it will be something they mention at any press conferences so it will be interesting to see how they slip it out
Or they will simply try to spin it as a bonus by saying something like "now you are always connected to what your friends are doing."