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They aren't saving money that way, trust me. Duplication costs alone (you can press DVDs at a rate of a few thousands per hour on one press vs writing out data at USB2 speeds through a vast array to the flash media alone is worth it).
Don't forget that Apple also ships more than just the restore media in CD form and that the Mac App Store right now can't accomodate all their apps.
I think the savings might be more than we know. Fair enough, I don't know enough about flash media - but I'm sure it's not that hard to figure out a way to mass 'image' the media. It might be that the memory chips are imaged during their manufacture, and then the chips - with the content stored - are put into the physical USB holder.
The cost of the media - DVD vs flash - is just a small part of the cost of getting the media to the consumer. There are costs associated with shipping the media, with handling the media, with inserting the media into it's packaging, etc etc.
Just for argument's sake assume that USB flash is the same size/weight as the DVD media - at least as far as the cost of shipping and handling are concerned. And assume that the cost of the DVD blank is 10 cents, and the USB media is $3 (I'm just pulling #s out of the air, for argument's sake). The cost of handling the media is, what?? - - $5 to $8 ???
So, now the cost of moving to USB media is in the range of between $5.10 vs $8.00
TO $8.10 vs $11. If Apple absorbed the cost of the change (ha!), they would lose at most $9 per system (System Restore, plus whatever 2 SW packages that the user would - on average - buy for the system from Apple.)
If the optical drive cost more than $9, then Apple wins. And we haven't even talked about what features Apple could include in it's laptops if they didn't have the weight and space of the OD to work around.
There's one other way that Apple can save money by going to flash media. The media is rewritable. With DVD media, once that SW is discontinued, the disk is trash. With flash media they can take all of the unsold restore disks, and reimage them with the update. Or .... If you buy a new Mac from the store, perhaps they would have a flash media updater station where you could bring your install media up to the current version before leaving the store. Just thinking out loud here.
Of course, they may decide to go to SD media instead of USB. I don't know enough about the technologies to make any predictions, though.
all of this imho of course....