What should a musician's Performance fee be for prestigious corporate gig?

Here’s the situation:
The event is Hannover Messe, the world’s largest industrial trade show. It is held in Germany with a different partnering country each year. This year the partnering country is the USA, and the USA wants to put on a cultural performance. Obama himself might be there (!!)
Here’s the question:
My good musician friend is interested in applying with his group and dancers, a total of 10 people. They’re supposed to ask their own price. What ballpark should they aim?

thanks for any help!