Quick, how much should I pay my friend for this?

A friend offered to fix my toilet. He’s taking out a part in the tank, then we’re going to Lowe’s to get the replacement, and also to get a new toilet seat, and then we’re coming back and he’s replacing the part and the seat. He’s already started working on it. Shouldn’t take more that 30-40 minutes of him working on the toilet I don’t think.

What’s a good amount to pay him for it? He said that I didn’t need to pay him but I said I would. Don’t have to pay him as much as a plumber but I want to make it worth his time, also he doesn’t have a lot of money and I want to help him out a bit.