Are there any corn price multiplier charts based on drought stressed low yield silage?

Lets say a field has been zero'd out, another estimated at 20 bushel, and another at 40 bushel. I'd just be trying to supplement dry cows in the 1250lb range to stretch the hay supply, but no one can really give us a figure that's based on such low yields. It would all be bought straight out of their piles 2-4 miles away depending on which pile/field I would buy from. We're long time neighbors, so just looking for a fair number. I feel buying directly from their pile as I need and when I need has significant value, so I'd definitely like to make sure he's paid for my convienence. Of course, the zero field is the closest pile and the 40bu field is the farthest.