I absolutely agree with Anne - a percentage arrangement, while still
sometimes used, is a good incentive to get fundraisers accepting all
kinds of inappropriate gifts on your organization's behalf.  Why not hire
someone really great, with a demonstrated track record, and plan to pay
her what she's worth?  I would certainly be leery of any organization
that offered me a compensation package involving a percentage - I would
worry about their fiscal stability, and about their understanding of the
development process.  There are other ways to track and encourage
performance.  For example, most units here at the University of
Virginia use the measure of numbers of significant calls (i.e. visits
with qualified prospects where the campaign and its goals, and that
individual's role in it, are discussed) made per year(150 to 300 being
the norm, depending upon supervisory responsibilities, etc.)  Of course
the total $$$ raised is also considered.  Raising really big money is
done through building relationships, and with the active involvement of
an organization's staff, board and volunteers.  It takes time, and it
isn't cheap, but it does work if done properly.

That's my two cents!