As of the time I write this, there's a sale going on for digital versions of various crossover series from IDW Publishing. I don't know how long it's been running, or how long these sales usually last, but there's some really worthwhile stuff there. Besides the fact that IDW is the publisher for licensed comics...so they have a great stable of their own to mix together...they've also done some crossovers with DC that are taken straight from a fanboy's Christmas list. Like this dream series:
Here's the link to the sale at IDW's site, and here's the comiXology link (the .com version) if you prefer that.