I agree, I make custom aluminum parts for the W-Body cars, and everything I sell is less than $100. It takes me a couple hours to make a set of parts, and I'm sure that intake can be made in that amount of time with the right equipment. In fact I'm going to try to make one, and if it works out I'll sell them for around half that price.
What you're missing here is the fact that the G8GT is a limited production vehicle (so far) and the amount of people wanting to modify them is even smaller. So, for say a few thousand units sold/shipped, for a decently sized aftermarket company, only making $20 on each unit isn't enough. Once you take into account "engineering" hours to design the intake to work correctly, going through multiple versions to make sure its correct, dyno testing, road testing, etc... You're talking about thousands of dollars of time spent (if the time was spent elsewhere making money, time costs money) to develop this product. Also, New Era seems to stand 100% behind their products, so you know they'll take back any "faulty" product with no questions asked, as a bad rep is money lost as well.
Personally, $380 is too steep for me to spend right now (its winter). Maybe this spring I'll convince myself, but most likely, I'll wait for them to put it on sale for a little cheaper. Once it falls around $300, I'll probably scoop it up. So, while I agree with you that its very expensive, its also a little more design intensive then a series of pipes and couplers, like every other CAI.
How do I come up with this? Well, I'm a product designer. My job is to factor design/test time into the total cost of the product.