Depends, there is a bus that shuttles back now and again, although you're given a hire car from NZ day 1 until the next set of cadets arrive from the UK - this can be around 3-4 weeks. After that period the hire car goes to the next set of cadets to arrive and you need to sort out your own car. Usually a group of cadets will buy a 2nd hand car together.. Average price range cadets pay NZ $ 1000-4000. Although this depends on what you're looking for.
A lot of cars are bought from cadets finishing in NZ by cadets who have just arrived.
You pay for your own petrol.