You do understand that aluminum is about the cheapest metal you can buy right? It is also a great disperser of heat...so its best for electronics. It is very difficult to weld as a result, but it is bendable...making it easy to fabricate into shapes. The polymer they are using on the 5C actually feels more solid than the 5. I had a 5, now I have a 5S. I don't like the colors of the 5C..but I went and picked one up and the materials (as always for apple) are TOP NOTCH. I know everyone has a stigma over "plastic" but there are many types of plastic. This isn't some cheapo clamshell.
I don't mind plastic at all b.c it means I have to baby my phone less since I use it naked. I could almost guarantee it is more expensive to make the 5 than 5C though. The anodizing process alone probably is cause for that.