Chuck, I've done some writing and editing of proposals as a consultant.  I
charge a daily consultant rate (based on my total remuneration from the
unviversity I work for--salary plus fringes--plus a little bonus for giving up
my free time), usually in day or half day increments.  But I sometimes give
non-profits or organizations I have a stake in a discount.

As a university, we have entered into a contract with the county we are located
in to provide some grant-related services: searching for and publicizning grant
sources, brainstorming with county personnel about possible grant ideas,
reviewing and editing proposals--no writing of proposals, however.  They're
paying us $5,000/yr. for the first two services and another $5,000 if we do any
heavy duty editing, in six month increments.  We're just completing our first
six months; everyone is satisfied so far.

My advice, for what it's worth, is to figure out what this work is worth to
your office and charge them accordingly.  Of course one of the considerations
is, how much do you want to do it?  And a part of that consideration is, how
interested are you in more work from this agency?
If you're not very interested, make your price high enough to interest you.
Regards, Bill Campbell
Director, Grants & Research
University of Wisconsin-River Falls