Steven Joshua Blue:  Experienced, Local, 30 years in Hawaii: w/ 10 years experience driving the Hana Highway (at least once a week, or more). 

If you have me drive YOUR rental car, then your custom, private tour costs you ~ $37.50/hour for an unforgettable experience. ($300 for an 8-hour tour. In YOUR car).

Note: The $300/8 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 $300 for our use of my vehicle for an 8/hr. day.
Figure $75/hr = ~$600/8hr+day.  ( for an unforgettable tour of Maui.)

After 8 hours, you need to expect to budget $75/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)