Mr Lance is correct. A performance fee would be a fee charged to perform (such as a singer giving a performance) and the fee quoted will be the entire fee, there will be no extras such as travel expenses.

Hi Lucka, "Flat" is often used with words such as"fee" and "rate". It refers to an amount that does not vary. If you are referring to investments, then it is a fixed payment made to an investment manager for generating positive returns.

Context? For a performance fee, it may be that the performer is the one being payed, rather than having to pay. Either way, "flat" means unchanging. It could be that the performer is being told that they don't get paid more for being famous, or bringing in more people, or more drinks being sold, or anything.