I estimate it at Q3 2005 rough guess.
I’m guessing Q2. Normally I would think that they would keep the prices high for a while longer than that to try to recoup thier development costs and make a decent profit. With competing blue ray formats being pushed through so fast to beat the other side, competing dvd formats still trying to out do the other, dvd-5 blanks dropping down so cheap, compression that makes good quality backups of a dvd-9 to a dvd-5 easy and cheap to find, it’s going to drive prices down. Dual layer just doesn’t offer any significant advantage to even come close justifying paying 20 times the price (much less even 5 times the price). If you are one of those people that has to have the latest and greatest and are willing to pay a super high premium for it, It won’t be long before blue ray stuff is avalable.
There are very projects that require a dual layer disc. They probably won’t be manufactured in sufficient quantity before blu-ray or hd-dvd comes out.