The plastic in the components are a minor cost. The major cost is the manufacturing process and transport. Its not uncommon for molds to cost $100,000++ each. Factor in the items such as fuel tanks are relatively small volume items,yet bulky ,and the unit price for a mold increases greatly. You've probably got $5( if that ) worth of actaul plastic in a fuel tank, the rest is mold cost, labour , transport and mark-up,etc.