I kinda see what you're getting at. This is not really an answer, but the fact that rental places like Blockbuster sell their excess "previewed" movies certainly applies downward price pressure. Also, once the demand slackens on a popular movie, there's need to lower inventory. And who knows, maybe the industry realizes lower prices = higher sales and less motive for piracy. Dunno, but I still like to backup my DVDs, for two reasons:
1) Preserve the original, of course
2) Strip out crapola and PUOs.
The price of Dual Layer media doesn't bear any relation to this that I can see. Prices will drop as initial investment is amortised, assuming the demand is sufficient.