I checked myself and found that an American made iPhone would cost approximately 5% more not 7% if parts were sourced from the same places.
And more information from Business Insider:
"if iPhone assembly were done in the US but the components were still sourced globally, the cost of making phones (currently estimated at about $230) would rise about 5%. However, if the components were made in the US (with raw materials bought on the global market), that would add an additional $30 or $40 to the cost of making the device, an increase that would then be reflected in retail markups".