summer 2006 or later...
Currently, the recordable kind (+R/-R) of double layer (or dual layer) discs cost upwards of $2 per disc!
Single layer DVD+R or DVD-R discs cost around 25 cents a disc or less on good deals!
The current price range of dual layer discs is clearly too high for movie DVD backups:
a) unless there's not enough hard disk space to backup commercial dual layer movie discs (until the prices come down)
or
b) the user doesn't want to use a DVD shrinking program to make the movie fit in a single disc
I'm guessing that dual layer DVD discs will start becoming cheaper sometime
after the middle of 2006, when the Blu-Ray and HD-DVD formats start showing up in movie titles (they would cost more than conventional DVDs, and Blu-Ray is expected to cost more than an arm and a leg when it's initially released).
The best place to buy dual layer discs keeps changing with time (as with any other product). Check regularly on sites like
http://www.edealinfo.com,
http://www.dealmac.com,
http://www.techbargains.com,
http://www.fatwallet.com etc. for good deals. For dual layer discs, currently there
doesn't seem to be a clear trend of prices going downward with time.