The reason DD cost as much is either they can, or due to regulation, or retailer. You know for instance Photoshop and many other packages like 3Dmax cost more than 50% in Europe, even when it is DD, than USA, matching retail price?
How else can you explain it other than to match retail prices in Europe, or because they can. Also some of the servers & companies that sell them are base in Europe, & you cannot use server & bandwidth as argument when the price difference are over easily 300 bulks.
I do not think however, the DD need to be cheaper than retail version, but they need to provide better value than they are now, for the masses to accept them. Be it family plan, or some form of trade in. I do not have the statistic, but I am fairly sure, being able to trade/exchange game hold more values for most people than the conveniences of not swapping discs or more storage space (especially your games are going to be sold back anyway).