One to two cents a task. Anytime you have a language agnostic task (identifying/classifying objects, etc), the tasks can be done very cheaply. Just make sure you do triplicate validation.
Language dependent/creative tasks run much higher (smaller worker pool, more brain power needed).
I've never used mechanical turk before and don't understand what you mean by language agnostic. I'd want someone to tell me that it's a "car" and not a 汽車. And I'd want to give the instructions for the task in English.
Language dependent/creative tasks run much higher (smaller worker pool, more brain power needed).