Great question — answering as the UphillTech team:
The overall cost really depends on whether you want to bring a vehicle or not. It is much more expensive to bring a vehicle. If you want to bring a vehicle, then your only reasonable option is the ferry. There are two ferry services: Interislander Bluebridge If you do not want to bring a vehicle, the cheapest way is probably to fly (although the ferry has better scenic views, unless the ocean conditions are rough). Flights between the main centres (Auckland, Wellington, Christchurch) will be the cheapest. Check: Grabaseat which is Air New Zealand's discount fare outlet Jetstar If you are renting a vehicle, then ask the rental company about options they have for ferry travel. Almost all...
Full answer by Greg Hewgill: https://travel.stackexchange.com/a/39776 (CC BY-SA 4.0).
Need it done for your business? UphillTech.