If there's one thing that Amazon Prime has taught me, it's that shipping charges are the only thing keeping me from spending all my money on dvd and blu-ray sets. That being said, here's some cheap blu-rays for you.
Firefly: The Complete Series [Blu-ray]
For some reason I didn't own this series yet, borrowed it from friends when I watch it. But at just eighteen dollars for the blu-ray I have no excuse for not owning this.
Fullmetal Alchemist: Brotherhood - Complete Collection One [Blu-ray]
Fullmetal Alchemist: Brotherhood - Complete Collection Two [Blu-ray]
If you like the Fullmetal Alchemist: Brotherhood, and haven't bought the individual collections yet, this is literally the cheapest you could get the entire series. Keep in mind that the individual sets were around fifty bucks a piece. Of course they're cheaper on Amazon, but if you want the whole series this is the perfect time.
One Piece: Collection One
Speaking of individual collections...I'm starting to wish I hadn't started buying the individual voyages for One Piece, because these collections are so very, very affordable. Curse my collector soul!
That's about it for now. If you find any other good deals on Amazon, let me know in the comments.