Here's an incomplete list of titles to which the 20% Funimation discount applies. I know for sure some of these are nowhere near historic lows. I'm not going to check each one to put it in here, but if you have any questions about a specific one, ask and I'll try to get back to you.
18 pages of listings. My eyes hurt.
A Certain Scientific Railgun S Season 2 Blu-ray/DVD(42.99 pre-discount)
Afro Samurai: Resurrection Director's Cut
Attack on Titan Season 1 Parts 1, 2
Attack on Titan Complete Season 1 Blu-ray/DVD($36.96 pre-discount)
Attack on Titan Season 2 Blu-ray/DVD($26.79 pre-discount)
Basilisk Complete Series Blu-ray/DVD
Black Butler Season 2 Blu-ray/DVD($32.46 pre-discount)
Black Butler: Book of Murder OVAs Blu-ray/DVD($24.79 pre-discount)
Cat Planet Cuties Blu-ray/DVD S.A.V.E Edition(14.99 pre-discount)
Code Geass: Lelouch of Rebellion Season One
Cowboy Bebop Complete Series Blu-ray/DVD($28.49 pre-discount)
Deadman Wonderland Blu-ray/DVD($19.96 pre-discount)
Death Note Complete Series Blu-ray($27.56 pre-discount)
Eureka Seven Complete Series Blu-ray($45.79 pre-discount)
FLCL Complete Series Blu-ray($24.44 pre-discount)
Fairy Tail Collections 1, 3, 4, 9, 11, and Zero. The collections between 1 and 9 might be on sale. Didn't check individually.
Freezing Season 1 Blu-ray/DVD($22.13 pre-discount)
Fruits Basket Complete Series Blu-ray($43.42 pre-discount)
Full Metal Panic: Fumoffu Blu-ray/DVD($17.99 pre-discount)
Ghost in the Shell Arise: Borders 1 & 2
Hellsing Ultimate: Volumes 1-4 Blu-ray/DVD($29.96 pre-discount)
High School DxD New: The Series Blu-ray/DVD($43.06 pre-discount)
High School DxD Born Season 3
Lord Marksman and Vanadis Blu-ray/DVD($37.39 pre-discount)
My Hero Academia Season 1 Blu-ray/DVD($42.99 pre-discount)
My Hero Academia Season 2 Part 1 Blu-ray/DVD($50.31 pre-discount)
One Punch Man
Ouran High School Host Club Blu-ray/DVD($30.72 pre-discount)
RWBY Volumes 3, 4
Serial Experiments: Lain Complete Series Blu-ray/DVD($22.99 pre-discount)
Shakugan no Shana Season 2 S.A.V.E Edition
Soul Eater Complete Series Blu-ray/DVD($29.96 pre-discount)
Speed Racer Complete Series Blu-ray($19.96 pre-discount)
Steins;gate Complete Series Blu-ray/DVD($24.22 pre-discount)
Tenchi Muyo: OVA Series
Devil May Cry Complete Series S.A.V.E. Edition
The Melancholy of Haruhi Suzumiya Seasons 1 +2 Blu-ray($40.00 pre-discount)
Tokyo Ghoul Complete Series Blu-ray/DVD($42.99 pre-discount)
Yu Yu Hakusho Seasons 2, 3
Only Yesterday Blu-ray/DVD($19.96 pre-discount)
Pom Poko Blu-ray/DVD($15.96 pre-discount)
Tales from Earthsea Blu-ray/DVD($14.99 pre-discount)
The Red Turtle Blu-ray($15.49 pre-discount)
When Marnie Was There Blu-ray/DVD($15.99 pre-discount)
Digimon Adventure Tri.: Confession
Dragon Ball Z Seasons 4, 6, 8
Dragon Ball Z Kai Seasons 1, 2, 3, 4
Dragon Ball Z Kai: The Final Chapters Parts 1, 2
Dragon Ball Z: Android 13/Bojack Unbound
Dragon Ball Z: Fusion Reborn/Wrath of the Dragon
Dragon Ball Z Double Feature: Tree of Might/Lord Slug
Dragon Ball Z: Broly Triple Feature($21.49 pre-discount)
Dragon Ball Z Double Feature: The History of Trunks/Bardock Blu-ray($21.49 pre-discount)
Inuyasha The Movie The Complete Collection
Naruto the Movie: The Last Blu-ray/DVD($14.99 pre-discount)
Pokemon: The Movies 1-3 Steelbook Collection($19.99 pre-discount)
Keep in mind that most things on Amazon are price-matched from other competitors, so if you don't have access to Amazon deals or are a part of some rewards program on some other website, I recommend checking other competitors' prices through Google.
So, as of late Amazon has been placing more and more anime related content on sale ranging from movies, OVAs, complete series, etc.
To see more deals for Amazon Prime members:
click on the link
click on "Anime" that is a part of "Movies & TV › Boxed Sets › Anime" directory which is located directly above the Cowboy Bebop box image
than in the "refine by" option on the left of the screen, click on the Amazon Prime image, so that only Prime deals are displayed
Some decent deals include: