If you find something someone lost, it takes time to track them down and get them their stuff back. Everyone's time is valuable. You are worth something, especially if you just did something like get their stuff back they wanted. If you don't mind volunteering your time, that's fine. But just because volunteers do it for free doen't make what they are doing valueless. If you need the money or it really was a chore, then it's fair to ask for a finders fee.
If the only driving force to do something honest and good is that you're going to get some kind of reward, that's wrong. We as people should do good not because we expect something, but because we simply want to do good. It's wrong if you expect something in return for doing something voluntary, as it says alot about one's character, revealing one to be selfish and displaying the 'What's in it for me?' attitude.