They only have 150 rounds total of the new 155mm Zumwalt gun type too. Only 150 rounds even exist right now. That's not even enough to fill up 1/2 of one gun turret's loading system. In fact the reserve storage rack holds 150 rounds, so they could fill that up, and leave the 600 slots in the gun turrets empty I guess.
Jolly, I've read that several places before, that it's due to the scale of manufacturing and the other DDG1000s being cancelled, that's mostly responsible for driving the price up on the rounds. It's unfortunate, as all the tests, the videos, docs, etc out there show that the system works extremely well, giving the ship the ability to stand off from very long range, far offshore, yet still be extremely accurate, with good effects on target.