The saintly thing is to offer it back to him for the price you paid. The "opportunistic bastard" approach is to take advantage of his misfortune to gouge him for all you can. And there is a huge gray area between these. It is up to you to decide where you fall on that line. Also, think about how you'd feel if you made a mistake (lost your wallet in a park, let one of your domains expire, missed a property tax payment, paid your credit card late, etc.) and how you'd like the other party to react.
My view is that it is fair to ask the price you paid, plus what you think is fair for your the time you spent researching and buying the name, and transferring it back. Last time this happened to me, I told the guy exactly what I paid for it (sent the Snapnames auction log) and told him how much extra I thought was fair for my trouble. The guy declined (the Snapnames price alone was more than $1,200), but he thanked me for the offer and said it was reasonable. So I got to keep the domain, and felt good about myself.
Note that this assumes it is a generic term domain. If it is a domain like foobarcafe.com or victoriastreetrecordstore.com, you should just give them back their name IMHO and shouldn't have bought it in the first place.