I'd wager (heh) Tooze is familiar with armament production. Germany still built something like 4k Leo 2s. Add to that about 1k of Leclercs, half that of Challengers, some Ariettes, and a bunch of other crap, there's room for an economy of scale, it's just not being used. And frankly, EU armies buying M1s isn't helping. In addition, you can put the proposition backwards, and say high unit costs are a sign of too few of them being built.
I'm wondering if he's maybe comparing older models. An M1A2 can come from anywhere between early nineties and today - L2A8s are brand spanking new. But even then, the PzH 2000 being six times more expensive then the K9 is just insane.
Once again, I'm not sure what conversation you think you're having, but it's not this one.