In an interview Sony gave to AV Watch recently, the company admitted it's going to "gradually end development and production" of recordable Blu-rays and other optical disc...
I'm not sure what country you're in, but have you tried Amazon? I found Verbatim 50 packs of 25GB for $40 and Ridata (usually good quality discs) for $28. If you want small amounts there's various 5 packs for less than $10.
It's getting harder and harder to make things smaller, but they are making things thinner now, which means they can layer them, thus increasing density.
Again, writable Blu-ray is a slightly different format from what you would buy with a movie on it. They are not putting Criterion movies on the same discs you could burn them to in a drive.