I have a different take on this since I don't derive a living from photography. I do it for fun, personal enjoyment, having the pictures, the challenge, having other people like pictures I show them, etc.
What I insist on is being properly credited. I get satisfaction out of knowing other people liked a picture from me enough to publish it, and the ability to show others "See, I took that". Therefore my aim is furthered by people using my pictures as much as possible, so I see charging for that only as hindering that aim.
I say right up front that any of my pictures can be used for non-commercial purposes, just that a credit and copyright must be associated with each copy of a picture. I explicity mention a school report as being non-commercial use I even encourage. I've had at least one of my pictures used in a PhD thesis that I know of. I state publicly that commercial use requires explicit permission and likely a fee, but that is mostly so I can so "no" in case it's a use I really don't like or don't want to give the impression I am supporting. So far I haven't said no, and it's unlikely something really obnoxious will come along so that I will.
The few times I have been approached for commercial use, I have also gotten the same sob story about no budget, etc. I didn't really believe it, but I also didn't care. I tell them I want a proper credit by every copy of the picture and if appropriate, one copy of whatever publication the picture is going into. So far nobody has objected to that.
Some of my pictures have ended up in unlikely places. One was a newspaper in Los Vegas that used a picture of a turkey vulture as a lead-in about corporate greed. And yes, they did give me proper credit and sent me a copy of the paper according to our deal. I got more of a kick out of that than any reasonable fee would have provided.
I understand and to some extent sympathize with what jrista is saying. I am one of the people he is complaining about that is cheapening photographs everywhere by letting mine be used for free. I have gotten hate mail telling me the same thing. Back when the internet was younger and the supply of online photographs much more limited, a professional photographer in Florida got upset at me and was quite rude about it. Until then I wasn't sure how to deal with commercial requests, but that solidified my decision to not charge. I still have no idea what a fair and customary price is anyway.
While I totally support anyone's right to charge money for their work, others equally have the right not to. I guess I am charging for my work, just that the compensation isn't money but something with more value to me in these cases.
I am curious what real professionals would charge, but it seems impossible to get a straight answer. I notice that none of the other answers gave any dollar amount. Answers like "what you think it's worth" and "only you know your market" are useless in demystifying this for someone like me. I have no idea what it's worth to someone else, and no, I don't know the market.
I have noticed that requests to use my pictures have fallen off sharply in recent years. I suspect that is because of the enourmous volume of alternatives out there. I guess that means pros have to compete on quality, the kind you can only achieve with some expertise and lots more time than the amateur with a day job has. If you don't, then it is only worth what the amateur is willing to give it away for.