I've had the same happen to me. In my case, a local bank wanted to use one of my photos that they found on flickr for their bank renovations. After conversing with the marketing manager I gathered they didn't want to pay anything for it. I asked some friends who also happen to be photographers and they encouraged me to charge for it. I'm glad I did. It was a bank after all. Ha!
Look at it this way: If they really want the shot, they can hire a photographer to take one just like it. So yeah, charge for it but be reasonable.