It's really not even that...
Current Verizon promotion is all phones, regardless of harddrive or model 4 and up to 5 are all worth 200. So, a iPhone 4 8GB model will fetch you the same as a iPhone 5 64 gb model.
AT&T the difference in trade in value for the iPhone 5 16gb is $10. 150 for 16gb, 160 for 64gb.
Gazelle is $10 difference as well.
So, the most you're seeing now is $10 and that's for going from the low-end to the top end or 16gb to 128gb. Nowhere near the $200 you pay in the end for the difference... So really, this shouldn't factor into your decision. Really should only be based on if you really think you need that space. I always use it, so I always get it. But the trade-in/resell is never a good investment for it anymore.