Above is a quick pricing analysis comparison between aluminum and steel. I changed the prices to reflect current pricing $1.43/lb for Aluminum and and $0.56 for 10-20 Steel, non-SAE grade.
Aluminum cost : $42.32
Steel cost: $32.43
Tot. Price Aluminum: $60,940,800
Tot. Price Steel: $46,699,200
Added Cost: $14,241,600
Weight Saving: 49%
3 important things to consider:
1. From that page, the weight savings of a move to Aluminum hoods from steel is about 30 pounds. That's too small to make a dent in EPA fuel ratings, which means an automaker would be spending $14,000,000 extra for no benefit. In order to really help fuel ratings, they might need to drop weight, say, 300 pounds. Then you're looking at $1,400,000,000 in added expenses.
2. Aluminum is weaker than steel. So you might need to replace 1 cubic foot of steel with (just for the sake of argument) 1.3 cubic feet of Aluminum to keep the same frame strength. That adds to your costs and reduces your weight savings.
3. Shaping the parts as you need them may cost more (or for all I know, less) with Aluminum and with steel. Some materials are easy to work with than others.
I don't mean to defend the status quo. It's not impossible that Detroit automakers have delayed a move to Aluminum and other lighter materials out of sheer idiocy.
But it's possible that cutting 5-10% off of the weight of a typical vehicle with Aluminum adds several thousand dollars to the production costs.