Answer:
Scrap Metal Prices are determined by the scrap yard you sell to. They will want to make as much money as possible. In a highly competitive areas, metals will be bought for closer to "spot" price, aka the price determined by the commodities market. If your scrap yard is looking to make a bigger profit from their purchase from you, then they will buy things for as cheap as possible.
The above is part of the answer but scrap metal prices are determined based upon Comex, LME and general economic supply and demand principles. Scrap yards will take this into account and then offer sellers a discounted price. Make sure you find a scrap yard that is paying close to these indices. Also, the cleaner and more prepared/sorted your scrap metal is will also determine price. For example clean copper will fetch a higher price then copper with insulation or general debris.