Steven Joshua Blue:  Experienced, Local, 27 years in Hawaii: w/ 8 years experience driving the Hana Highway (at least one a week, or more). 

If you have me drive YOUR rental car, then your custom, private tour costs you ~ $8.35/hour for an unforgettable experience. ($400 for an 8-hour tour. In YOUR car) ($400/day = ~$8.35/hour/person for 6 people.)(Includes custom label, chilled, filtered, bottled water)

Note: The $400/8 hr. day gets JUST ME, your guide, driving YOUR rental car. You provide the transportation.
If I we use my Town & Country minivan, then you need to budget another $400 for our use of my vehicle for an 8/hr. day.
Figure $100/hr = $800/day.  (Cost per passenge:   ~$16.65 per hour for an unforgettable tour of Maui.)

After 8 hours, you need to expect to budget $100/hour for the extra hours of the tour.
Of course, this may include pickup and drop-off at wherever you stay while on Maui: hotel, condo, etc.
Mahalo nui loa. (Thank you very much.)

If you find yourself at the Kahului Airport on Maui, Please: strongly request/demand Airport Taxi #62. Or,
For a ride to your condo, hotel or resort, Call: Surf Taxi @ 808-870-9974 and book a taxi.

For Surf Taxi - Maui tours: $100/hr for 1–6 people)