That was one of the GTA4 developers gripes with the xbox 360 - fitting it onto a DVD. Imagine having to swap disks when you tried to drive to a certain part of the island!
the game and world don't take up to much space at all, its all the high def motion vid, theres no issue with duplicating the world data on each disk so you can travel anywhere, but at set points through the game after a cutscene after you've done whatever task you switch disks and proceed without issue. Haven't they actually complained more about the slower bluray transfer rate on the sony than the size problems on the xbox 360. at the end of the day sony went with a low speed bluray to keep noise down which is fine, xbox went with a higher speed dvd drive, they aren't massively different on either, both have limitations to design, as the gpu, cpu, hard drive and network speeds dictate how the game is designed aswell.
as for price, dvd's started to hit i would say a majority of home pc's when they hit the £70-100 for a reader price, and you've been able to choose a bluray as optional for a long time now, only recently cheaply.
toshiba, amongst everyone else will jump on bluray now, which means larger mass production of drive parts and lasers, which is the main cost/supply issue at the moment, so that will trigger a massive drop. more companies will produce drives now so the competition between various bluray companies will be BIGGER than between the limited number of bluray/hd-dvd drives available now as so many people were waiting for a winning before releasing a product.
readers sub £50 within2-3 months max, writers at the £100 mark within 5-6 months max.
DL dvd's WERE £10 a disk on launch, now as said you can get 10 for £4-5 , bluray will hit similar prices which will mean 250gb's for £5 of disks, a TB for £20, 1/5 the price of HDD storage and protectable and portable. mostly we need some faster drives, 4-8x drives so they don't take all damn day to write
Media for dual layer drives didn't drop until people had the drives. only took 6 months to go from £10 a disk to £2 a disk, and another 6 months to go to 50p a disk.