How are instructors compensated for teaching a Massive Open Online Course (MOOC)?
As @JeffE said, every department is different. There is remarkably little information available on the compensation offered to the instructors teaching the courses. A quick search revealed this article which states that this professor at Duke University doesn't receive any compensation for teaching MOOCs, including no reduction in teaching load, or any teaching assistants. See also this scathing article which states that professors teaching MOOCs are grossly underpaid--no hard numbers anywhere though.