The sales levels of anime adaptated from manga are basically a crapshoot. Not so much Light Novels. Though I still haven’t compiled at the 2012 data, but one particular bit of information popped out of the 2011 sample that it deserves its own individual mention. It’s best described as 2 rules of thumb:
1. No LN series that had at least one week where a volume sold 20,000 copies in that week (disregarding cumulative totals) that went on to sell less than 4000 disks per volume. Only one (Mayo Chiki) sold less than 5000.
2. No LN series without at least one week where a volume sold 20,000 copies sold more than 4000 disks per volume. Only one (Kore wa Zombie Desu Ka?) sold over 3000.
Obviously, this isn’t an exact measure of how strong an anime adaptation will perform. Kyoukaisenjou no Horizon and Infinite Stratos sold many fewer LNs than Ha Ga Nai prior to their anime debuts, but both sold an average of 10,000 more disks per volume. And it’s not an absolute line: there are enough LN series that had peak weeks just under 20000 that some would surely hit a figure slightly above 4000 disks per volume if we increased the sample size. Ultimately, that changes the text in the above rules from “No” to “Barely any”. This is simply the caveman’s graphical representation of a trend that’s not exactly a secret: popularity of the source material is a much, much more important factor for Light Novel adaptations than it is for Manga adaptations.
This doesn’t necessarily mean that adaptations of unpopular Light Novels are a waste of cash. They can produce boosts in first-week LN sales at the ~5,000-10,000 copies per volume level (Zombie, Gosick, and Bento all received respectable boosts in this range) that last for several years, which can offset some costs provided the actual disk sales are at least borderline.Though naturally, the same average boosts for LNs with popular anime tend to be bigger:
When it comes to adapting Light Novels that are mid-tier sales-wise, you don’t see the kind of explosive upside that turned Blue Exorcist from a 40k manga to a 400k manga in 6 months. Which is actually kind of an issue that merits discussion. There’s practically no chance of getting huge returns from LNs in sales purgatory; is it worth it to adapt best-case-break-even series when one really can just cherry pick the lucrative, sure-fire franchises?*
*There is the argument that such a sales strategy does promote the overall brand of the LN publisher; Dengeki Bunko does have a hand in most of their adaptations. It’s damn near impossible to quantify the actual effectiveness of such a strategy, and I’m a bit of a skeptic in this regard. This isn’t at all like Shueisha/WSJ’s farm system, where One Piece provides them obscene amounts of cover to produce the next big hit – an unsuccessful 10-episode anime carries a much higher price tag than a 20-something chapter manga. It’s possible the culture of seniority in LN magazines is similar to that in Weekly Shonen Sunday, where attention is given to the present lineup of authors who have worked long enough [to “earn” an anime] rather than the long-term outlook of the company. I have no idea how true that last sentence is, which is why this is in a footnote. And in general, feel free to point out any aspects of LN culture I’m missing. I’m trying to be thorough through all this, but the last LN I read was Tokyopop’s release of Gosick in 2011 and, beyond having seen a shit ton of sales charts lately, I’m way out of touch with what’s happening in that community right now.