Ok, so Monroe just thought that was $2.5 million PER YEAR, and not the actual $2.5 million total it actually was. It is still a bonehead mistake, but at least not completely inexcusable. If he had to rush the article at all (not saying he did, or even guessing, but just hypothetically here), that at least makes sense. There are so many other things writers must concern themselves that often times something like that, even as glaringly obvious as it is, will still slip through the cracks. And even the cracks of his fact-checkers. Remember, writers have those as well. Don't give the all the blame to the name.
I'm sure he'll remember to check those things next time, though. I imagine seeing that mistake in print will be fairly embarrassing.