Even at $1299 I don't see Apple going this low considering they sell a 27" at $1000. If they offered the same quality and options as the 27" display (usb, thunderbolt, hdmi, audio, ethernet, wifi) I would expect 2560x1440 (1440P) resolution. This TV would be brand new to a brand new market of 1440P and I would expect something like this to run $2000+ for a 50".
If it's reasonably priced, I'll probably think about grabbing one. Based on the current Thunderbolt display, if they go down that route, a 42" TV's going to cost a lot more than the market norm.
42" at the same price as the Thunderbolt, and has the same colour quality, £799 would be a very reasonable asking price.
I'll look forward to seeing how this develops.