Is this through the cruise line or on your own? Kauai is a beautiful island, but the roads are narrow and you could find yourself delayed if there's an accident or anything else blocking the road. Be sure to allow plenty of time to get back to the ship on time.
We found one for around $135 each and it's an island tour and a riverboat tour. We haven't booked it yet, but i chatted with their reservation department on line and this is a tour in a van, i was told 15 people was the maximum. Its not an SUV, but it is almost $100 less than the tour you are talking about. My big thing is the bus tours, i dont care what the price is, i've done some before, and i just don't like them.