Hey I was wondering how much is an extra baggage cost from Shenzhen to Auckland ?

It depends on the airlines policy. Which airlines will you use? It would be better if you contact the airlines to double confirm the details. I think it depends on the pieces, weight.

hainan airline, i try contacting the airline but it doesnt work. i want to get another 23kg baggage

For China-US route, the Hainan Airlines allows 2 pieces of luggage for economy class, each one should be witin 23kg. The total dimension of each piece should be within 158cm. If you want to add one more piece, the cost should be USD200/CNY1300 per piece.