Hmmm, that's tough. It seems to me that at yard sales I've been to where quilters were parting with their stash, they sold it by the pound and it ended up costing about $4-5 dollars a yard, which was a good deal. It would be hard to know how much fabric yardage you were getting in if you are getting it in scraps and if it adds up to $15.00 worth of value.
Maybe visit the store and ask how they came up with their price. They may have already measured out a pound of scraps and can tell you approximately how much yardage you'd be getting for the price.