Not only that but I'm guessing the 64GB will hold its resale value much better than the smaller phones.
It generally evens out; a quick look at Ebay confirms this. They go, generally, for a hundred or so over the previous, which is the cost of going up with upgrading.
There are other factors that play into it as well, but you generally come out even. Plus, I would gauge that people would be less likely to spend more on a year old phone when the difference is 16 gbs.