MGS4 was on a dual layer disc (aka 50GB blu-ray disc). Of course the price won't change, if it did people might be reluctant to buy new games.
MGS4 would not have worked on a 25GB blu-ray disc. I think that was the most detailed game I have played. The environments, chracters, and whole story was simply amazing. Wonder if MGS Rising will be a dual layer disc; probably not considering it is also on 360 which only uses single layer DVD discs.
Most 360 games are on Dual Layer Discs. Its just that since blu-ray holds way more when its a dual layer (obviously).
That leads me to believe that MGSR is not going to be as large of a game as MGS4, since it will be on 360 which does not support blu-ray discs. Hope I'm wrong about MGSR though.