The only legendary resource you can get is Fiery Brimstone, and yes its quite rare - it sells for around 150k on the AH. The recent patch (1.0.3) has lowered the chances of getting one from salvaging a magic (blue) item, and increased the brimstones needed for some popular crafting recipes.
Always salvage lvl 60 items, exquisite essence is worth around 1100 gold and could go up as demand increases. Most of these will sell to vendor for a pittance, and even the 'good' ones don't go for remotely close to 2k.
Note that lvl 59 blues will give you Hell resources and not Inferno level. The exception is rings & amulets; which have a different formula - don't know what, but I can confirm that you get exquisite essence (the Inferno version) and a chance of iridescent tear for lvl 59 rings.
Also, only a level 60 legendary is guaranteed to salvage into a Brimstone - even if you find a really crappy one, always remember its lowest worth is the going price of a Fiery Brimstone. One more thing to note: there is a thriving market in the Trading forum on battle.net, where you could potentially get a little more for your essences than listing on AH after 15% fee is considered. I suggest being careful and only selling to people with multiple vouches however, or even better just hoard the items for your own future use ;).
Edit: You can read fbueckert's post on salvage levels for more info. As mentioned by Tater596 and Dave, lvl 59 follower tokens also salvage to Inferno essence.