From Reddit:
DLC summary mentioned above.
There was a thread about this last night, but it seems to have been deleted. I figured it was worth posting again since a lot of people (me included) have been dying for a sale and they so rarely ever happen.
How to use:
Open Origin client and go into game details
Click "Buy Now" on any of the DLC and see a screen that tells you how many BioWare Points you need
Click "Continue" and you'll see this screen: http://i.imgur.com/AFTezX6.png
Click "Promo Code" (code is ORIGIN30) and you can enter it.
You can't add all the DLC to a "cart" and check out all at once, but you can buy each DLC individually and apply the code each time. I purchased 6 separate items from both Mass Effect 2 and 3 yesterday (for $46! my cheapgamer heart hurts) and I was able to apply it to all of them.
GameStop Trilogy Sale $10.19:
If you're on PC, you might be interested in getting the trilogy (Mass Effect 1, 2, 3 and DLC for 1) to go along with the DLC sale.
What's the cheapest way to get everything? Are the "Deluxe" editions a good buy?:
I think the cheapest way to get all the games and story DLC is to avoid the ME2 and 3 Deluxe Editions; they either don't include any story DLC or are much more expensive than the trilogy on sale. Buy the trilogy for $10 ($30 regular price), buy the ME3 Digital Deluxe Edition Upgrade ($10, includes From Ashes among other things; From Ashes is $10 by itself), and then individually buy the other DLC you want. Check out /u/rowan72's excellent DLC FAQ for overviews of each DLC if you're undecided.
Steam versions vs. Origin versions:
This old comment says that ME 1 and 2 Steam keys should work to activate games in Origin. Look for "CD Key" in your Steam library.
DLC summary mentioned above.