I just noticed this today (because I luckily forgot to turn off notifications from the app), Comixology is having a DC Essential 25 Graphic Novel sale, with the digital collections selling for $5.99. The sale ends 1/2/14.
Pretty good deal, and if you happen to have a phone or tablet that supports it (like the Nexus 5), they also offer their comics in HD versions. There's some pretty good ones on the list.
I went to compare prices with Amazon, and the Kindle Edition seems to be matching the prices for these 25 collections, so you could also get them there. They don't have an HD version, but chances are if you're getting a Vol. 1 of something, the following volumes will be less expensive there.
But for one-shot collections like Watchmen, Batman: Year One, Kingdom Come, or All-Star Superman you could get it on Comixology now, and even if you don't have something with an HD screen, there's a good chance you will later on. I'd heard good things about All-Star Superman so I might get that one.