Considering Kojima says that TPP is hundreds time bigger than GZ, i don't see why they priced it the way they did. I love MGS, but paying 2/3 of the price of a full game, for what seems like a demo, doesn't do the franchise justice.
PS4/XOne
GZ = £29.99
TPP = £49.99
I was one of those people that played the MGS 1 demo and the MGS 2 tanker demo over and over again, because they had that element of infiltration, where you could find your own path through the missions. From what Hideo has said, GZ is much like this, with a few side ops chucked in. But again, that's relating it to that of a demo of a game, so this isn't going down well with the fanbase.
I've already pre-ordered it, and it only cost me £20 for PS4 using discount codes. But that's still fairly steep for a two hour story.