This is probably the biggest factor for most people.
The iPhone 7 costs $550 for 32GB of storage, or $650 for 128GB.
The iPhone 7 Plus costs $670 for 32GB of storage, or $770 for 128GB.
The iPhone X costs $1,000 for 64GB of storage, or $1,150 for 256GB.
Keep in mind: If you buy an iPhone X, you should also buy AppleCare+ ($200), since out-of-warranty repair costs can be incredibly costly.
With AppleCare+, iPhone X screen repairs costs $30, and other repairs cost $100.
Without AppleCare+, iPhone X screen repairs costs $280, and other repairs cost a whopping $550.
To be all the way clear: If you don't buy AppleCare with your iPhone X, and its glass back breaks or shatters, you are paying $550 — the cost of an iPhone 7.