Hi
First. English is not my native language, sorry.
Second. I think i found something, nothing big, but i don't want to be a lurking leecher.
There is fish called "Magic Eater", You can find it in Dalaran sewers so its cheap like hell on AH. "magic Eater" sometimes gives You random food buff but can give You a debuff called "Wild Magic", preventing spellcasting and combat actions for 45 sec.
You can easly remove that debuff using Noggenfogger elixir (from Tanaris), wich transform You into skeleton. Demonology Warlocks can use Metamorphosis talent, Iron Boot Flask works too.
So, Magic eater + pack of Noggenfogger elixir = cheapest food buff (on my server).