How to calculate price adjustment?

I contracted for replacement of asphalt shingle roof on a Victorian house in NJ, USA, at $18,000. I was told by all who gave estimates that under existing shingles were cedar shakes, which would have to be removed and replaced with plywood, and that this contributed greatly to the cost. Upon tearing off the roof, it turned out that shakes were only on a small part of it and that most of roof under the old shingles was tongue and groove. The roofer bought 120 pieces of plywood but only used 10. He wants to credit me $12 per piece of plywood X 110 ($1320) and reduce price by only that amount. There is a written proposal but no signed contract. Three questions:

1. Is $12 per plywood piece accurate (I don't know size, but I assume some roofing standard)
2. More important: didn't he escape tons of work by not having to remove shakes or install
110 pieces of plywood?
3. What would be a fair adjustment? I'd like to reach a fair resolution, so any input would be appreciate.

Thank you.

 

When the military was using all the plywood in Iraq, the price was way more than $12. Go to any Home Depot & check the price. He saved on a ton of nails too.

On the labor, I would say that he saved 3 days salary to his workers. Let's say he had 10 guys at $100 each. That's $1,000 per day. That's how I would calculate it.