I have set on several review boards for other schools, and the best thing I have received has been dinner afterwards (normally the test have been in the evenings). On one all day Saturday test, I was given $100, a nice certificate of appreciation and dinner. The dinner was the best part.
If you are having a BBQ after the testing, invite them to the table, serve them first, and that should be fine. If you feel you need to add money, give it to them discretely.
I do ask that you actually listen to their input. In most, if not all, of the test I set on the panel on, it was just for looks as my comments, and the comments of the other guest black belts on the panel, were not listened to if they were negative in any way, and the only way a person should sit on the panel is giving their professional opinion!
Yes, I know that they know more about the student and what they have progressed from/to, but if you are in a system such as Judo, Kendo, TSD, TKD, there are semi set requirements of standard (even if most schools don't follow them), that the person should be graded against!