What is a reasonable referral fee?

I'm curious, what exactly constitutes a reasonable referral fee in the world of cryptocurrency and finance? Is it based on a percentage of the transaction amount? Or is it a flat rate? How do professionals in this field typically determine what's fair and appropriate? And are there any factors that can affect the referral fee, such as the complexity of the transaction or the size of the client's portfolio? I'd love to hear your thoughts on this!