Thierrymv, my girlfriend and I spent a month in a campervan in January 2013 and had a blast. We rented ours from Freedom Campers (
http://www.freedomcampers.co.nz/) and had the 4+1 Pathfinder. We picked it up in Auckland and dropped it off in Christchurch, which is a well-traveled path.
We enjoyed it, but it was a little too small for even just the two of us...I'm not sure we could have handled it with a young one with us.
As others have mentioned, you'll be paying a premium for traveling during the peak of the kiwi summer; if we'd been a month or two later in our travels, it probably would have cost us 1/2 to 1/3rd as much.
Gasoline was also expensive; we figured that our costs ran roughly $0.30/mile and we were filling up the tank ($120 at a pop) roughly every three days or so.
One way we cut costs was by utilizing New Zealand's great network of public campsites owned by their Department of Conservation. These are basic, unpowered, and really cheap ~$10/night. We tried to alternate between spending one night at one of these sites and one night at a Top 10 Holiday Park (which are everywhere, and which will sell you a $40 membership card that saves you 10% on every night you stay, as well as 10% on the cost of the ferry between the North and South Islands). The Top 10 campgrounds usually ran about $40 a night.
And although you didn't ask, let me take the liberty of giving you some recommendations: on the North Island, the glow worm caves of Waitomo, the Thermal Wonderland in Rotorua, and the art deco in Napier. On the South Island, whale-watching in Kaikoura, heli-hiking in Franz Josef glacier, and penguin-spotting in Dunedin.
Finally, just for fun, here's a video we made about our lovely campervan, New Zealand Exciting. Thanks for indulging me. :)
http://www.acoupleontheroad.com/2013/04/07/saying-goodbye-to-new-zealand-exciting/
