Buburimu is always packed lately so I decided to try farm blood in the maze of shakhrami... I cleared out the room full of leeches (there's like 20) DRG/THF and a THF with TH2... not a single bird blood, just a couple of fiend bloods. After a minute killing we decided to go back to Buburimu... Onzozo drop as good as Buburimu though, but at lvl 62 it's slow killing for me.
Just a note: to my knowledge, this only drops off of the leeches in Onzozo, not the ones in Gustav. Which makes sense, seeing as there aren't any birds in Gustav; Onzozo has cockatrices.
Anyway, this is a very common drop, from my experiences. I get one almost every kill with TH2.
The few singles that had sold last I checked sold for 10k/piece, and I sold the first 2 stacks in the AH history for 100k each, but that price will most likely drop when more people know where it comes from and how common a drop it is.