Get to know the scrappers,and shop their loads. If they find out you will pay cash for a certain item, they will bring you more...
You need to know what they get for an average truckload. Once you see how hard they work for what they make, you can make them a fair offer on what you want:
A bike weighs very little. To get the most for it, they must strip the aluminum parts off. They would rather have an old BBQ.
They will also trade. I swapped a dead motorcycle battery for a bike once.