This is your correct answer. Market value varies in different parts of the country. What scrap yard would give him is a starting point accurate for your region. He might give you a slightly better deal per pound since you would be loading the lead and buying his equipment. Either way nip several at random with a pair of fairly large diagonal wire cutters (dikes) lead is soft and will "nip" the edge easily. Zinc is harder. As someone pointed out zinc & steel "trash" can run from almost none to 50% of the weight. Grab a handful from a few places, nip 20 WW's if 2 are zinc you are looking at roughly 10% non lead WW's If 10 are too hard to nip then you are paying for 50% non-lead WW's
Also good to check if scrap yard will sell you lead and if so at what price. This lets you know if the scrap yard might be a future source and at what price. Not all yards will sell to the public. Worst case you can buy clean ready to use ingots in the swapping and selling forum here for about $1 per pound.
You need to have an idea of material costs anyway and by getting those up front you can make a better decision on if his WW's are a good or bad deal for a certain price. For the seller it is scrap yard or you unless he does more work so you only should have to be close or match the price scrap yard pays for lead WW's coming in. However at much over 50 cents a pound you might as well buy ready made ingots from the S&S forum. Between hauling, sorting, propane, and casting time you pay over 50 cents a pound and the time and other costs will make $1 a pound for ready to use ingots a good deal.
There is a reason I don't sell WW ingots. I pay too much around here to make $1 a pound profitable. I do some for self and look at it as a cost saving measure that allows me to accumulate more stash for same cash but sure not worth shipping for the small amount I would gain from the sale.