That was my question some time back. I have the multiple volume set. I did wind up getting the At Home and I think it is a good addition. Also, when relevant it cross-references to the more detailed sections in the larger volume, which is helpful. It is not in my view a subset of the larger volume, it is mostly new (though recipes such as stock are the same) and more home cook oriented and less science and background info.
Also I picked up At Home at B&N over the holidays for $65 when they had it at 50% off. At $65 I think it was a good deal. At $103, the Amazon price at the time I write this, I still recommend it if you are inclined toward learning about "modernist" techniques.
Incidentally, I don't recall a book's price bouncing around so much. When I first saw it at Amazon it was about $140, if I recall, which was list. Then it went down a bit. Then it went up. Then B&H had it cheaper. Then B&N put it on a huge holiday sale. Now I took a look today at Amazon and it's at $103. It's like a volatile stock. Maybe you should buy options on the book instead of the book.