The thing you have to remember about pewter is it is not something comes into scrapyards at a regular rate, so they are not able to turn it around fast and that is why they likely don't pay big bucks for it. Odds are it will take most yards quite a while to fill up even one gaylord box worth of pewter to actually ship it because it is not a common item, and most scrappers don't separate it from their loads.
Also, like others have said there are several varieties of pewter with different chemistries. I don't think there will be too many yards throwing around big numbers for pewter.