Some of my biggest ones:

Christmas 2006: Got 1st 5 seasons of 24 for $20 each and Six Feet Under: The Complete Series for $115.

Freaks and Geeks: Yearbook Edition for $127.50

This past fall: Sopranos Season 4, 5 and 6 Part 1 for $30 each at Costco.

Don't recall the date, but it would probably be the day I bought fifty anime titles from RightStuf. Couldn't pass up the "25 titles for $100" deal on the Geneons.

A couple DD sales ago I had a $500 or so order.

I track stuff by when I got it. On 11/21/05 I got these 43 movies/DVDs (some were part of sets) for 412.12:

Rage: 20 Years of Punk Rock West Coast Style
Looney Tunes: Golden Collection: Volume Three
Ben-Hur
War of the Worlds: The Complete First Season
Blood Sisters (1987)
Harum Scarum
Viva Las Vegas
Jailhouse Rock
It Happened at the World's Fair
Spinout
Speedway
Bride of Re-Animator
View from the Top
Pirates of Silicon Valley
Scrooge
Good to See You Again, Alice Cooper
Heaven Can Wait
Lock, Stock and Two Smoking Barrels
eXistenZ
Xena: Warrior Princess - Season One
Blondie: Greatest Video Hits
Kentucky Fried Movie
Youth of the Beast
Sorcerer
Creature Comforts: The Complete First Season
Cry-Baby
Magnificent Butcher
Concert For Bangladesh
Bullet for Sandoval
Star Trek: Nemesis
Better Off Dead
Fisher King
How the Grinch Stole Christmas!
Scrooged
Decameron
To Hell and Back
Zombi 3
Zombie 4: After Death
Zombie 5: Killing Birds
Drums Along the Mohawk
Ghostbusters 1
Ghostbusters 2

05/06/03 - Twilight Zone Gold Collection. 49 disks for ~ $300.

I did quite a few large orders back in 2001-2003. Looks like the largest one was $996. Here's a screendump from the online order list:

It wasn't my biggest purchase (its two different purchases), but by far the biggest bunch I received in the mail was 10/22/07.

O Lucky Man! (Warner R1)
Under the Volcano: Criterion Collection
Breathless: The Criterion Collection
Days of Heaven: The Criterion Collection

The above were all DVDPlanet, the below are all one order from Amazon UK.

My Way Home (Second Run R2)
Underground (Artificial Eye R2)
Rosetta / La Promese (Artificial Eye R2)
Eric Rohmer Collection (Arrow R2)
Kaurismaki Collection Volume 1 (Artificial Eye R2)
Kaurismaki Collection Volume 2 (Artificial Eye R2)

A grand total of 25 discs received in one day - ten items, twenty two films.

I spent over $700 in a single deepdiscount order in Nov. 2006 sale.

On my birthday in 2005 was the day I bought the most DVDs I ever bought:

2 free DVDs:

Raptiva medicine information DVD
Play Your Game (Nike soccer DVD)

20 DVDs from the Dollar Store (the most I bought at one time);

Incredible Story Studios:"Out Of This World"
Incredible Story Studios:"Be Careful What You Wish For"
My Global Adventure Vol. 2
Protectors Of Universe
Space Thunder Kids
Defenders Of Space
Beauty And Warrior
Duel Of The Tough
Borderline/ Mutiny
Ballad Of Andy Crocker/ Absolution
Second Coming Of Suzanne/ Mitchell
Hercules Unchained/ Sword Of Lancelot
Cyrnao De Bergerac/ Angel On My Shoulder
Strike Force
Row Your Boat
Dead Silent
Someone Is Watching
Wild Rio
Species Hunter
Shark Quest

DVDs I bought from Ross clothing store:

The Lucy Show (28 episode, 4 disc set)
The Three Stooges Collection (2 disc set)
Dragnet (20 episode, 2 disc set)
Love That Bob (16 episode, 2 disc set)
The Beverly Hillbillies Collection (50 episode, 5 disc set)

Total price that day:$53.95

I don't remember prices, or dates, but for online (or through mail) purchases it would have been my initial Columbia House order (the first time, I'm re-upping) 10 or 12 titles depending on what the deal was at the time. For in-store purchase it would be in Dec 006 I got Seasons 1-4 & 6 of Charmed (5 titles). I have also gotten 6 titles at one time as well. I don't remember titles or date, but I think I once got 8 titles in-store, but that is not-confirmed so I'll stick with 5 or 6 at one time my biggest haul