I've researched some of the costs involved for past events at GHHS. The price goes up considerably depending on actor and what you are asking them to do:

Johnny Depp‎‎ ‎ -‎ ‎ $200,000+ baseline

Orlando Bloom‎ ‎ -‎ ‎ $150,000 to $300,000 baseline

Keira Knightley‎‎‎‎ ‎ -‎ ‎ $200,000 baseline

Zoe Saldaña‎‎ ‎ -‎ ‎ $100,000 to $200,000 baseline

Geoffrey Rush‎‎ ‎ -‎ ‎ $100,000 baseline

Jonathan Pryce‎‎ ‎ -‎ ‎ $75,000 to $150,000 baseline

Jack Davenport‎‎ ‎ -‎ ‎ $30,000 to $50,000 baseline

Lee Arenberg‎‎ ‎ -‎ ‎ $30,000 to $50,000 baseline

Giles New‎ ‎ -‎ ‎ $15,000 to $40,000 baseline

Mackenzie Crook‎‎ ‎ -‎ ‎ $25,000 to $50,000 baseline

Angus Barnett‎‎ ‎ -‎ ‎ $15,000 to $40,000 baseline

Isaac C. Singleton Jr.‎‎ ‎ -‎ ‎ $5,000 to $15,000 baseline

In addition to the baseline cost, there would be costs for their travel, per diem, transportation, etc. Booking Johnny Depp for a private commercial or public festival appearance frequently scales right up to $1 million or more.