Hmm, looking at the auction house over an extended period shows that nightmare blues sell between 200-500 gold and the nightmare essence can be purchased for around 50gold. Hell and Inferno essence is similar around 50-70 gold which would be worth even more. The rare ingredient is a bit more from the auction house 200-400. But these items sell for a lot more anyway.
So the moral of the story, Never salvage your gear. Sell it to a vender and buy essence off the auction house. This is of course depending on what happens months after launch but the to speculate the increase in time should only increase the items on the auction house.
Similarly keep an eye out for items with a buy out of a lower cost than the vender sell price. Using the buy out search option you can make a lot of quick easy wins.