If your university is willing and able to fund the advisory committee it will
strengthen your grant proposal significantly.  NSF, like all funding
agencies, is impressed by the commitment of real local dollars to
grant-funded projects.  If that is not a feasible option, I would consider
using local folks rather than folks from elsewhere for your advisory
committee.  Saves a great deal of travel cost.  As to honoraria, if your
committee consists of a few faculty from your institution and some
visitors from close by, you may not need to pay them at all--though it
would be nice to do so.  I usually compute honoraria by deciding what
sorts of folks I want to include, finding out from one or two what they
would do it for, computing what I can afford, and assigning some
relatively arbitrary figure.  But it will be one rate for all; who has time to
compute different rates, or to pay different rates when the time comes?

Good luck.

Bill Campbell
Director, Grants & Research
UW-River Falls