I’ve never paid anything but a flat daily rate in Bali for a private tour driver. Usually in the $40-50 per day range, including gas. Doesn’t matter if it’s one person or six. Unless you’re only paying $10-15 a person.

Do not let a tour driver take you to expensive gem, silver, batik or wood shops unless that is your express desire. They get a commission if you buy or a spiff for coming by.