I am still in the process of assessing which instance is going to be most lucrative for farming. At the moment, I'm leaning towards the Hellfire Ramparts. There are a couple of reasons for this.
- Ramps has more boss fights, and correspondingly a greater number of blue drops, (and thus a higher gold yield) than the Blood Furnace.
- It is a relatively small instance, and can thus be cleared comparitively quickly. With Beast Mastery and Viper up at all times, (even while fighting) I can do a single run in around 30 minutes. I have no idea about other people, but for me, that is extremely fast.
- Given the close proximity of Thrallmar, it is also the closest instance I know of to a mailbox, and repair/resupply facilities. This means that if I do not mind using lower rank ammunition and water, I can do repeated clears without having to hearth back to Dalaran to resupply.
- Although per-run yield is about the same as Scholo, (100g, give or take) there is one very important difference. In the case of Ramparts, that 100g comes entirely from material that can be directly sold to vendors. In the case of the Scholomance, a lot of it was dependent on the Auction House, and whether or not Kali was feeling kind where the RNG was concerned, in terms of Dark Rune drops.
I do, however, still need to level Mining, and intend to do so at some point.