What do you charge your boat passengers for gas?

I was curious what most people do for gas money for the boat?

I've always been the type that doesn't say anything about it and they usually give me $20 to $30 dollars for gas and a lot of times nothing for gas. My friends who understand how much these boats suck fuel understand more than others. Now that wake surfing is the primary activity on the water fuel consumption is getting worse. Having a boat payment and having to constantly pay for most of the gas is starting to get old. Is it fair to ask your passengers to pay for the gas? or Just ask for a flat dollar amount per person? Lets be honest a lot of the time the boat owner is usually the one driving the boat while most of the others are enjoying themselves surfing, wake boarding, or partying. Share some of your process with me so I don't feel like I'm being unfair or unreasonable.

Thanks