We really don't know that the iPhone 5C costs less to make than the iPhone 5.
I do think that this was Apple's way to maintain their flagship premier device and also PERCEPTUALLY a "budget" phone - that also can be more whimsical (more color options, etc - which will appeal to many.
Now they have the best of both worlds. They don't "tarnish" the elegance of the "iPhone" but instead of a "c" series. It's no surprise there's no black or gold iPhone 5C...
At the end of the day - I think Apple created two products with plenty of profit margin to make a killing.
I likewise think the "C" products are a great idea (one of the few for Apple in the past couple years).