Hello, I just found out today that you get 2k rep each Lava core / Fiery Core turnin instead of 500 that you got before, So if you are farming towards the "exalted" title. Then buy out a few from the ah, they cost around 20g each on my server.
You turn them in inside Blackrock Depths at this npc "http://www.wowhead.com/quest=6643"