I can't completely agree with the "Universal did better with less" line of reasoning. Part of the budget overrun on Pandora has to be due to the unique nature of this construction project. The floating mountains with built-in waterfalls, the unique features that will be in the environment of the land (glowing things and whatnot), the extensive rock mountain wall that will effectively hide the entire show building (not just part of it like Hogwarts) of two combined attractions. Yeah, Universal didn't spend that much on Diagon Alley, but it also contains nothing innovative in either the attraction(s) or the architecture. Crooked buildings, a coaster attraction that's similar in scope to the Mummy, and a train with TVs in it. It's hardly ground-breaking in a way that would inflate the money it cost to design and construct. A relatively "off-the-shelf" coaster and a train. Beautifully themed, of course, but not the kind of epic scope that would result in unexpected budget inflation. Basically the Mummy coaster with 3D projections and a short train ride, and some quirky-skinned polygonal buildings that are a few stories tall. The rides have a been-there, done-that kind of feel if you ignore the Pottery skinning. And not that I'm saying it's a bad thing, but budget-wise, I can understand why it was relatively cheap.
I'm not saying Disney is "efficient" with the money spent on this project, I'm just saying that comparing it to Diagon Alley on a budgetary scale is an iffy comparison. Diagon Alley is beautiful, and I'm sure it's even more effective on a Potter fan, but Pandora is going to have mountains with waterfalls floating over a hundred feet in the air, rivers, trees, glowing plants, and (possibly) some animatronic fauna and interactive (and/or glowing) walkways and other elements. I've never seen anything like that in a theme park. And the two attractions are of unknown final impact. The boat ride could be so beautiful that nobody will care that it might only be 5 minutes long, and Flights could be breathtaking. Might it be the first 60p digital, 3D, full-range motion simulator? I haven't been following much lately, but as far as I know, it could be. You can't really judge the efficiency of the final budget until you see the final product.