This answer from Frank Datzer at Allied Vaughn:
DVD duplication is done by using towers of burners to duplicate large amounts of DVDs at a time. This produces exact copies or DVD-Rs of your original. Replication actually uses a glass master to “stamp” new DVDs. This method dramatically lowers the price per unit at high volumes — 1000 DVD will run about $1.25 each from a DVD-R master.
This includes all glass mastering fees and 5 color silk screening on the disc (5th color is a white background or “flood” coat.
DVD duplication machines with ink-jet printers can be a good idea if you only need a couple dozen…they can be expensive to run - ink runs out quickly and the design is not waterproof - no sweaty hands! Kind of slow too — dependent on the amount of data on the disc.
__________________
Frank Datzer
Allied Vaughn
www.alliedvaughn.com
fdatzer@wi.rr.com
DV Enlightenment
After Effects PowerStart
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Motion PowerStart
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SteadyTracker
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Skycrane Jr.
Tiffen Filters
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FireStore FS-4 Direct to Edit
Edirol Video Mixers
Reflecmedia 
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Lowel Ego light
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Apple Computers
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Digital Speakers
Video Processor / DV Converter
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Sorenson Squeeze
Primera DVD Duplicator
Matters of Light & Depth
Editing Techniques with FCP
APTS : DVD Studio Pro 4
APTS: FCP Advanced FX Book
FCP Visual QuickPro Guide
Let us not over-look the most important part - quality. When dealing with a replication (1,000 )project, especially DVDs you’ll be able to seperate the competition.
Ink jet is “sprayed” ink period. Insert/Panels/Tray Cards are OFFSET
printed not run through some laser jet with “all most” Offset quality.
Creating a glass-master enhances the process, CIRC pre-tests your master in finding additional errors most “duplicators” overlook.
We find the biggest choice clients have is a 10 day turn-time. That we put on clients for ill-preperation. Rush charges are applicable because services like Allied Vaugh have a reputation to maintain within the industry. It only shows their commitment to quality.
How old is this article? Dual Layer DVD-R 9 drives have been around for a few years now.
http://en.wikipedia.org/wiki/Dvd9#Dual_layer_recording
Eric
Thanks Shep, I edited the article to correct it. It was written in 2003 I think