It's $20 on the PS3; consumers are essentially paying a $10 premium to see the game on the best platform, presumably because Konami knows people will pay it and presumably because there's a smaller potential market on the new consoles (PS4 sales of Ground Zeroes say otherwise) due to the smaller install base.
$20 isn't much of a value for a prologue considering that this was the kind of thing that used to be packed in with a game previously to boost sales of an average title. $30 is even worse considering the types of titles that are available at that discounted price.
But... the point of packaging Ground Zeroes and selling it as a separate product is essentially as a service for the die hard Metal Gear fans who would rather not wait for Phantom Pain. I'd say it also helps finance Phantom Pain, but that game will sell enough copies that Konami could have just given away Ground Zeroes as a promo attached to another Konami title (which would mean paying $60 for the game just to play the prologue).
So for Metal Gear fans and collectors who want another MGS item or just don't want to wait for the full game to be finished, it's worth it.
For anyone else, I could see why they would be understandably upset if they paid $30 for something that was just a teaser of a full title game.







