Prices for cash sales in the scrapyard are generally based on the "what the punter is daft enough to pay" scale. And if you really do go in and buy one short-end of 25mm brass bar, the transaction will attract the inevitable Nuisance Charge.
Buy as much as you can sensibly afford on each visit and always haggle. Never accept the first price. There will normally be quite a bit of room for negotiation. Remember though, that guy may have a dirty face and dress like a flood victim, but he's probably the shrewdest business man you will ever meet. Haggling with someone like that is not always easy, but just because he's the only scrappy in the area who will even talk to you doesn't mean you have to get your leg lifted.
Unless you are buying enough to need the weighbridge, or want a receipt, then the money from these sales generally goes straight into his back pocket anyway. As long as he's getting more for the metal than he paid for it, he'll be happy.
Good luck.
one_rod.