Some of the titles like The Dark Knight Returns are still only available for pre-order. While nearly everything is $9.99, there is also some strange pricing. Grant Morrison's All Star Superman is $9.05, for instance. Best deal here seems to be Fables Vol. 1 for $7.99.
Incidentally, I was shopping on Comixology this weekend after I got my Fire and I found that they offer each issue of Kingdom Come for $1.99 each. This means you can have all four for under $8, as opposed to the $9.99 price at Amazon for the collection.
I personally don't love reading comics on the Fire, but I'm interested to see what differences, if any, there are in reading comics with the Comixology app from reading the graphic novels bought directly from Amazon.