You can set a single price for an entire phone case category. In the below example, ALL iPhone Slim cases i.e. iPhone 6/6s, 6/6s Plus, 7/7s Plus 8/8 Plus, X/XR, XS/XS Max will have the same $20.00 sales price.
Tough cases will generally retail for $3-5 more than slim cases because they have extra protection with a rubber insert. And Flexi cases will retail for a little less. Make sure to price your cases appropriately!