I truly think this is what we will see.
iPhone 7/7+ respectively:
32GB = $649/$749
128GB = $799/$899
256GB = $899/$1049
Base model stays the same price but storage doubles, the next two tiers each go up. 64GB-> 128GB = $50 increase and 128GB->256GB = $100 increase. It will follow the same exact model of iPad Pros.