C.E.
What I was always told is you figure that what ever your meal should cost is what you should give as a gift...However a few months back I saw this question on Miss Manners or Dear Prude, and the response was...there is no set way to figure and that is a gift not a payment for your dinner or your evening. I would look at it this way: How close are you this person? How much time do you spend with them? and then; How much do you have to impress your family...although it is not necessarily their business you know most families if you are a real scrooge or very generous you are going to hear it from a aunt, uncle, grandparent,etc....
Can you afford the $100 right now? If you are close and it's not to h*** o* you then that should be good. Also how close is this cousin to your children...
Good luck
Chelle E