Is there a way to see how long he has owned the domain? From there, estimate a yearly cost to him (12$ * years registered) + 10% profit and start from there. That seems reasonable.
You can use whois to see when the domain was registered. Since all the other extensions weren't registered I'd assume this is more of a brand style name and was registered fairly recently. It is hard to assign value of a domain without knowing what it is. Also, most people who sell domains are looking for the sales to compensate for the rest of their portfolio that don't sell. So registration fees + 10% wouldn't likely work as an offer.
The economics of owning a lot of domains pays off when your portfolio covers the cost of all registrations/renewals. The average churn rate on big portfolios is 2-3%. So simply offering a 10% on top as profit doesn't work. The average sale price is ~$2000.
