It depends on which discs you are trying to back up (obviously), and you may just have a preference for the types of releases that are being released on dual layer original discs.
These tend to be the major releases, the newer, mainstream "hollywood blockbuster" type of fare. This is because the industry knows that extras sell, and that a certain segment of the market is also prone to buy based on the picture quality (and if less compression is used on the transfer, the film might take up more than a single layer). There is a somewhat sizable market for the "Superbit DVDs," as we have seen. And, there is a much larger group that is fascinated with extras, so much that they will buy a release from Best Buy or Circuit City merely because they are offering an additional "bonus" disc with even more and more extras, regardless of the obvious fact that these are the extras that were so ho-hum that they did not even make the cut...
Independent films tend to end up as a single layer DVD release more often. Foreign films tend to end up as a single layer DVD release more often. Older films, and even classic films tend to be end up as a single layer DVD release more often. The family that purchases a lot of these might well have posted here that they almost never see a "dual layer" store bought disc.
Also, those films which were released early on, and have not had any re-release are going to be on a single layer disc, since early on this was the only thing coming out of the factories.
Now, you may be frustrated by this... but here are a few thoughts that may (or may not) be important:
-you can always try to eliminate any stuff which you deem NOT important on a backup, which sometimes can make the entire thing fit with no compression. When this works, this means no compression, the backup looks as great as the original.
-you can always try to eliminate any stuff which you deem NOT important on a backup, which will always mean you are left using less compression. In many cases, depending on your needs you end up with very, very little compression. A lot of people do not the DTS sound, if they do not have the DTS-capable system. A lot of people do not need the foreign language soundtracks, or at least those in four languages they do not speak. A lot of people do not need the subtitles, or at least those in seven languages they do not read. A surprising number of people realize that they can also eliminate the cast bios, and "Making of Featurette," and the "Making of 'The Making of' featurette. Many people eliminate the deleted scenes. It is only a backup, right? Some people find they can live without a director's commentary. Many more people discover they can live without the commentary by the second unit photographer, the guy in the bunny suit in scene 3, and the assistant sound editor, who also helped on some of the Foley. Those storyboards might have been the reason you bought the disc in the first place, but maybe seeing them the first four times was enough, especially since you still have them on the original disc, right? And, especially since the movie is only "Halloween 15: Jason Does Dallas."
-you can always choose to "split" the dual layer discs to two blank DVD single layer discs. This is actually my own personal preference. The only real compromise is that we get up once and change a disc, but this can be pre-set at a comfortable chapter break, and it has become the "bathroom, replenish the snacks and refreshment, and change the disc" break. No compression, the backup looks as great as the original.
-you can do any of the above, or even mix and match from the above, while you wait and watch those dual layer blank discs begin to continue to fall in price. In 18 months, they have gone from $13.99 per disc to $13 per disc to $10 per disc to $8 per disc, and then $7 per disc. Then, it was $6, and as you have noted, $5 per disc (especially in retail stores.) The going rate for the very best ones -- (Verbatim DVD+R) -- on-line, right now is between $4 and $4.60 per disc. There were two recent sales where they sold for $3.99 per disc. They will hit $1.00 per disc, and I recall not too long ago when single layer blanks were much higher than that.
Of course, by the time that day comes, propbably this winter, or early spring, we will suddenly begin the tiniest speck of interest in a new, seductive enticement... Blue-Ray... HD-DVD... and, of course, the inevitable Blue-Ray writers. And burners. Not to mention, the inevitable HD-DVD writers. And burners.
All of this, while we eagerly anticipate the Special Edition re-release of the classic 1960s sit-com "My Mother the Car" on DVD, along with that special bonus disc featuring the documentary "Body Shop: How The NBC Censors Made Me Tone Down Those Huge Headlamps for PrimeTime Television."
Keep it all in perspective,