What you say makes sense. If, from reading the emails, a reasonable person would assume that Coz agreed to the $100 price, then he should stick to it. But, I'd have the see the emails before I could say that he actually agreed to that price. It would be nice customer service if Coz were to give you a partial refund, but I can't really say whether or not he should give it to you.
In the future, I'd just advise making 100% sure that you and the seller are both in agreement on the price before you send any payment. Be happy that you're only talking about $25 here and not a major chunk of money.