What’s the best wording to ask Booking Agent what a Celebrity’s fee is?

From my experience, I would just provide them all the essential details about the event including: about the event, date, city, venue, and sometimes attendance numbers from previous events. And within that same email, would ask what the booking price is. Sometimes they would provide an exact number, other times they may ask you to submit an offer. In this case, I would either just submit the offer or ask if they could provide me a range.

Agents aren't trying to pull a fast on you, after all if you don't make a profit then your unlike to do more bookings so its really not in their best interest to overcharge you. Its a negotiation, so both sides should benefit. This is more of a question on you wanting to get the best deal, and this is dependent on how much research you did on the artist and their previous shows + performance, in addition to your ability to negotiate.