Now, I will give some better understandings on how specific sales figure represents the success of late-night TV series. All the sales of Movies are out of question, as the scale is different and it also has various other source of income from filming itself. Success of so called 'late night animes' are totally dependent of the DVD/BD sales.
Here is the list;
<<DVD+BD sales and its respective meanings>>~1000: The lowest late night animation can reach, no failure can be less than this. However, it is common sales for most of main stream kid or shonen animes which does not consider DVD sales from the start anyway. (Example: Munto, Natsu no Arashi, Fairy Tale, A lot of noitaminA)
1000~2000: Failure. It's not as bad as above, but still very sad. (Ef - a tale of melodies, Dance in the Vampire Bund, Kaichou wa Maid-sama! Mitsudomoe)
2000~3000: It has some fans, but didn't sell due to popularity crisis or original breaker. (True Tears)
3000~4000: This is about break-even point for most of animes, aka Manabi line. Thus, it is still along the line of failure (Maria+Holic, Ga-rei Zero, Arakawa Under the Bridge, Baccano!)
4000~5000: This is also about break-even point for most of animes. This is an average achievement. (Higurashi no Naku Koro ni,
5000~6000: This is a line where you can expect 2nd season. Good achievement for any late night animes. (Amagami SS, Sekirei ~Pure Engagement~)
6000~7000: I consider this as semi-hit, but it varies. This is a bit ambiguous line, since you can't really call it 'a big hit', but it is still a good result. Sometimes, there are animes where you had big anticipation, but ended up being not good. (Highschool of the Dead, Sayonara Zetsubou Sensei, Seikon no Qwaser, Baka to Test to Shoukanjuu)
7000~8000: Semi-hit. Staffs should have party. But still ambiguous to call it extremely successful. (Katanagatari, Pani Poni Dash!)
8000~10000: Very good result, but I consider it more of a cult hit, where otaku plays a lot of role. (All Hidamari Sketch series, Fruits Basket)
10000~12000: Huge success for otaku oriented late night series. Fujoushi-hit series is often included here. Some people who aren't otaku might have heard of it. (Striker Witches, Durarara!, Natsume Yuujinchou)
12000~15000: Most of hardcore otakus will own series with this sales. This is cut -line where you can call 'otaku hit' or getting 'original content boost', although it is highly opinionated. (Gintama, Aria)
15000~20000: This is already at the level where those who don't usually by Animation DVD/BD get into the play. Massive hit. It is still heavily influenced by otaku buying power and media-mix boost. (Working!, Full Metal Alchemist!-Brotherhood, Tengen Toppa Gurren Lagann, Sengoku Basara)
20000~30000: Die-hard fans and votaries come into the play. However, it already passed the level of only appealing otakus, but any animation/manga manias. However, most of the series have hard time being consistently on this level of the sales through the volume. (Nanoha, Fate/Stay Night, Lucky Star)
30000~50000: Marked on animation history. Legend. (K-On!, Suzumiya Haruhi, Angel Beats!, Fullmetal Alchemist, Macross Frontier, Ano Hana)
50000~100000: Almost impossible for late night animation. Gundam, Bakemonogatari and Madoka Magica are the only TV series that reached this point. Godly sales. However, it is common for OVA or Movies (Kara no Kyokai, Gundam SEED, Bakemonogatari, Mahou Shoujo Madoka Magica)
1000000~: Only movies or OVA (Evangelion, Gundam Unicorn)
Edited: someone in ANN had modified list that is 'similar' or referenced from mine. So, I'm adding a bit of his opinion too, to give better understanding. You can also look at his one too, but remember, my list is much simpler version as it does not take account to the first volume sales, but average volume sales.
Edited: Added fucking Madoka Magica and Ano Hana
ringOtamegane's Last Words
meh.... it sickens me when someone tries to interpret sales data without actually knowing what is success and what isn't.