OK...
But my personal feel would be around 15k (if 300F2.8 RRP is 12k).
I doubt that it's gonna be that expensive, if it's a 500 F2.8, yeah maybe 15k is reasonable, but 500 F4? IMO, unlikely.
300 F2.8 + 1.4 TC could perform 420mm F4, I think to make people buy for the size and weight of this thing, i would say it should cost a bit lesser than the 300 2.8.