Book One of The Iron Trial by Holly Black and Cassandra Clare is just $2.99! And it's still in hardback.
This is categorized as "upper middle grade," so it can be considered "young adult" as well. I have the hardback (because some stores had signed copies), but I'm considering getting the eBook anyway. And I want Book Two to come out soon.

(I'm reallly interested in what happens to the MC next because of what he learned at the end of Book One..)