Week 3 of DC's Convergence sale includes more 70s/80s/90s goodness and ends 4/27/15. I believe this is the first time that any Alan Moore Swamp Thing has been on sale.
Batman and the Outsiders 1-21
Green Lantern 180-200
Sage of the Swamp Thing 1-28
Omac 1
The Flash 340-350
https://www.comixology.com/Convergence-Week-3-Sale/page/6258

Another Teenage Mutant Ninja Turtle collection sale: 20 books (10 from the newish IDW ongoing series and 10 from older runs) ending 4/27/15.
https://www.comixology.com/Discover-the-Teenage-Mutant-Ninja-Turtles-Sale/page/6277

The huge Dark Horse sale starts today. 50% off everything, a stackable extra 10% code coming via their newsletter, and a $2 credit to everyone on their list.
https://digital.darkhorse.com

Bandette #1 has been free before, but it's free again to celebrate its Eisner nomination.
https://www.comixology.com/Bandette-1/digital-comic/27559

To celebrate the new Submit page art (?), all issues and collections of Doc Unknown are 50% off with code UNKNOWN. Unknown duration, no pun intended. The collections and singles appear to not overlap, anyone know for sure?
https://www.comixology.com/50-Off-Doc-Unknown-Use-Code-UNKNOWN/list/6412

Image is selling their Eisner nominated books at 50% off with code EISNERS, sale ends 4/26/15.
https://www.comixology.com/2015-Image-Eisner-Nominee-Sale/page/6360

Several new foreign sales, and some new #1s for that Marvel True Believers sale on the main sale page.
https://www.comixology.com/comics-sale

New freebie.
https://www.comixology.com/Avengers-Magazine-2015-1/digital-comic/215434

The huge sale from the best publisher of comics since the 80s is still going. 50% off everything, a stackable extra 10% code of DHD4YEAR from their newsletter, and a $2 credit to everyone on their list.
https://digital.darkhorse.com

Marvel is continuing their Avengers sales with a one day sale on the 1998+ run.
issues 0-84 and 500-503, $.99 each or all in a bundle for $60
https://www.comixology.com/Avengers-Sale/page/6361
https://www.comixology.com/Avengers-Earths-Mightiest-Heroes/bundle/915

Here is the page for all the current True Believer sales:
https://www.comixology.com/Marvel-True-Believers-Sale/list/6134

From Hexagon, the first Silver Shadow volume is on sale for $.99, unknown duration.
https://www.comixology.com/comics-sale?list_id=6444&lang=

Weekend Eisner sale from Oni Press on their three nominated titles.
50% off volume three of Mermin.
https://www.comixology.com/Mermin-Vol-3/digital-comic/135386
$.99 singles and 50% off the collection of Meteor Man. The collection appears to add 12 extra pages.
https://www.comixology.com/Meteor-Men/comics-series/23605
$.99 singles and 50% off the collection of I Was the Cat. The collection appears to have 15 extra pages.
https://www.comixology.com/I-Was-the-Cat/comics-series/22705

Another huge sale from Caliber. 50% off everything with code CALIBER, ends 4/26/15.
https://www.comixology.com/Caliber-Sale/page/6396

You already own these singles if you bought any of the several recent Dynamite bundles, but a Jungle Girl sale ends 4/27/15. 1 freebie, 10 singles at $.99 and one omnibus at $8.
https://www.comixology.com/Jungle-Girl-Sale/page/6288

From Top Cow, a Stjepan Sejic sale ending 4/27/15.
13 collections priced from $4-7
a bunch of singles all $.99 except as noted
Death Vigil 1-6
Artifacts 0-16, Origins
Aphrodite IX vol 2 1-11 (1 is free)
Aphrodite IX/Cyberforce 1 for $1.99
Aphrodite IX: The Hidden Files
Broken Trinity 1-3, Prelude
First Born, 0-3, Aftermath
Angelus 1-6
Rat Queens 9
IXth Generation 1-2
https://www.comixology.com/Stjepan-Sejic-Sale/page/6306

So now Comixology is releasing freebie codes via their podcast, the streaming site/app Twitch, and that cool new Twitter app Periscope.

SOULE from Twitch gets you his first book for free, normally $5.

Do you know a cheaper site than Comixology?

I am interested in the Spawn series but it seems very expensive.

They sell it the range of $2 to $12 for a single issue, so all 250+ of them will cost $500 to $1000 or even more which is way beyond my budget.

Thanks. Got me a free Strongman Vol. 1 thanks to your post. Is it worth reading?

BTW, I need to finish reading Aphrodite IX vol 2 to decide whether it's worth buying. I like the art, but I'm not yet convinced it's an interesting read.

You can use coupon code FB to save 50% on your first order, I think that's still live. Also, it "pays" to wait for sales; they recently had a Spawn 1-250 bundle for ~$200. There arent really any cheaper sites (or more expensive) than Comixology.

No single issue should be $12, that was probably a collection.

Comixology pricing is basically the same as print issues when they both release, but then gets cut in half after a few months. They also have frequent freebies, sales, and bundles which bring the price down under $1 per issue.

$2 an issue is pretty fair, and is the best you are going to get unless you roll the dice and wait for sales.

Yep, as Undeadcow said, there are very few options for legal digital comics. Marvel and DC have their own sites/apps but they are just Comixology fronts basically. Anything you buy at their sites will show up in your Comixology account too. Image has their own site separate from Comixology with similar pricing but different deals occasionally. Dark Horse sells on their site /app only and have great sales. Thrillbent is another place for a few smaller companies, and 2000ad stuff is limited to their own site iirc.

Marvel has a today only Prepare for the Age of Ultron sale.
https://www.comixology.com/Prepare-for-Age-of-Ultron/page/6367
Mighty Avengers (2007+) 1-6
Annihilation: Conquest 1-6 and Prologue
Avengers (1998+) 0, 19-22
Moon Knight (2010+) 1-12
Avengers (2010+) 1-6

New Marvel collection sale with 10 Avengers collections at up to 80% off.
https://www.comixology.com/Avengers-Collection-Sale-Up-to-80-off/list/6451

Oh Thank you. I had to get use comic pricing then.
Thank you, I see that reading comics will be an expensive hobby.

It can be expensive or pretty cheap, like any hobby.

I own about 15,000 digital comics. About 3000 were free, most $.99, and thousands cheaper than that due to bundles. You could have bought all those new for $60,000 physical or digital, or be patient in either format and average less than a dollar a book.

Well, I am just not used to it, when compare to novels or games that could last me days, 30 comics maybe last me hours.

And wow, 15K comics, you must at least spend 10K on them.