Just because you get more GB on the disc doesn't mean it's cheaper to make.
The process to make it is more costly due to being new and needing new equipment, not as many made. More produced=less production cost.
If one BD 50 cost less than one dvd wouldn't devs use that on the ps3? but you say they stick to BD25 which you say are cheaper to produce than 2 DVD9's.
Which is it?