"Does the insurance carrier allow you to, fax the vob form to them? How long does it take the insurance carries, to return the completed vob form?" - You would have to check with the carrier on these questions.
"What is a fair fee to verify benefits, the form has 12 questions?" - This sounds like you are referring to a specific form for one carrier. I would determine how much time is involved (total, filling out form, faxing and tracking) and base your fee on that time.
Michele