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 hard on you then that should be good. Also how close is this cousin to your children...
Good luck
Chelle E