The obvious big-dollar line-item would be the mushroom cloud budget. They will most definitely include a reenactment of the Trinity test, and likely the attacks on Hiroshima and Nagasaki (and possibly the non-nuclear attack on Tokyo). These will be a different variety of nuclear explosion than we've seen in movies before, since the Trinity test really was a controlled experiment, analyzed in real time a thousand different ways. Knowing Nolan, he was probably disappointed that he couldn't arrange filming access to a real nuclear test, but I guess even Universal can't buy off the IAEA. At any rate, it'll take some spectacular effects work for them to do justice to nuclear explosions.

But other than that, it's a pretty standard-issue war-era historical drama, as far as filming logistics go. From friends at Los Alamos, I do know that they've done a great deal of on-location filming there and probably a few other places. Los Alamos is a bustling little town nowadays, and in the Manhattan Project days it was extremely remote. So it probably wasn't easy to revert it to something historically convincing.

It sounds like they have a huge cast, which makes sense given the huge variety of people who played important roles in this story. So that's probably a part of it.

It occurs to me that they might be covering a long span of time, as well. Oppenheimer had entanglements with progressive politics and (at times) the Communist party, going all the way back to his grad school days. And late in his life these entanglements came back to him in very ugly ways (there's even a ready-made villain, but I'll save the spoiler). Personally I think that story, while profound and fascinating in itself, is a side-story compared to the Manhattan Project, but it's easy to get across dramatically, so I bet they'll put some effort into covering it.