I would try to figure out how much per person they are paying for the event and give at least that with a little extra thrown in as another good rule of thumb.
That is a great suggestion! I helped with the planning, so I know they paid $23 per adult, and $10 for kids. So, our family cost them $79. We gifted her $200, which I hope was good, because it was all we could really afford!
Anyway, everything was great! It was so heartwarming to be a part of the traditional celebration. After we finished the private family/close friend dinner, before the party started, her Mom and I helped her change in to her ball gown, and put on her makeup.
Then she presented a doll to a child, and her Dad too off her child-hood shoes and helped her put on her grown-up shoes.
Then the party started! The teenagers & kids danced all night and had a blast, and us adults had a great time watching, and catching up with each-other.