I think that a lot of westerners fail to realize the impact of the "otaku as superconsumer" effect on the Japanese media industry, in part due to the way that anime is priced in western territories. In Japan, anime blu-ray volumes are tremendously expensive; right now the latest volume of Madoka is selling for over ¥9,000 (>$100), and that's a fairly typical price for a single volume containing three 25-minute episodes. In the west, $300-400 will get you several hundred 45-minute episodes of Deep Space 9 on DVD. In Japan, $300-400 is the amount that a hardcore fan can be expected to spend on the blu-rays for a dozen 25-minute episodes. On top of this, there is merchandising galore; CDs for OP/ED, insert songs, character CDs, along with physical goods like figures.
On top of this, sometimes hardcore fans will buy multiple copies of their favorites to inflate sales numbers. For an extreme example, see this man who bought 5500 copies of a single AKB48 CD: http://blog.esuteru.com/archives/3347822.html
To put things into perspective, the Bakemonogatari blu-rays sold around 50,000 copies per volume during the release window, which catapulted it to the status of "most commercially-successful anime since Evangelion." Based on this, it's not hard to see how a base of several thousand really dedicated fans could be enough to justify an anime production.
On top of this, sometimes hardcore fans will buy multiple copies of their favorites to inflate sales numbers. For an extreme example, see this man who bought 5500 copies of a single AKB48 CD: http://blog.esuteru.com/archives/3347822.html
To put things into perspective, the Bakemonogatari blu-rays sold around 50,000 copies per volume during the release window, which catapulted it to the status of "most commercially-successful anime since Evangelion." Based on this, it's not hard to see how a base of several thousand really dedicated fans could be enough to justify an anime production.