How do you determine the worth of a service provider that tells you to use your own discretion to pay him or her?

[King Belieal]

I recently contacted a friend to come help me play with my band. The guy is a close friend but is well known within the city. He didn't negotiate a fee with me for his services. He told me to my discretion to pay him. I don't want to dishonour him by paying him an amount that is less than his worth.

How would I determine the worth of a pro that tells me to use my discretion to pay him?

 

[eLdavis]

In cases like this, i simply put myself in their shoes and think of what i would want to be paid if i where the one who carried out that service.