We went there for Christmas and rentals were expensive. I guess its the high season there so they can go down in price at other times of the year. I rented from Avis but there are lots of options it seemed. We looked at the camper vans but it didnt seem to save us much money vs a hotel + car and honestly, I didnt see all that many places I would have actually wanted to park and stay.
Since they drive on the LEFT side of the road, I made sure I got an automatic since I didnt think I would be able to reverse learn how to drive a manual in the mountains and while on the opposite side of the road.
Gas was well over $2.00/litre. Most Chase cards dont charge foreign transaction fees which was nice. We drove all over the South Island for 10 days. Feel free to hit me up with any specific questions.