We were just there on motorbikes (so couldn't ride into the park at all). We didn't check, but our guidebook suggested N$400 per night for two people to camp inside. On the outside bush camping is impossible (or at least very difficult) on the 35km stretch from the main road to the east gate.
However you can camp at Onguma, a classy lodge on the right JUST before you reach the park gates (at the east end). It costs N$135 per person per night and you get a private bathroom, hot water and power.
If I was in a 4WD I would still spend a night or two at the camps further west in the park.
Vladivostok - Denmark 2008... Finished.
Denmark - South Africa 2010-2011... Finished.