There's this . . . thing going around on the Innertubes, part of a virtual scavenger hunt apparently, where people contact writers they've never met and ask them to write a 140-word flash fiction without offering to pay anything. And a surprising number of writers oblige them.
That's the writer's option -- you're allowed to value your time and talent however you wish. I have no beef with those who decide to do it just for fun or in order to do something nice for a stranger.
But it did occur to me that this might be a good time to provide you the simple rule by which you can decide whether or not to honor a request to write something for free. This happens periodically. Somebody contacts you to ask if you'll write a story for an anthology to benefit Bedbug Relief or whatever. It seems a worthy cause. You scratch your head.
Here's what you do: Calculate how much that story is worth. Ten cents a word is a good guess, if it's just an okay story. (You don't want to donate one of your best because then you're throwing away all chances of being nominated for and possibly winning a major award.) You lose all the intangibles of exposure in Asimov's or Analog, but what the heck. Life is too short to worry about intangibles.
Now ask yourself: Would you write a check in that amount for this particular cause?
If the answer is yes, writing the story makes eminent sense because the story's value is pre-taxes. It won't add to what you have to pay into Social Security and you don't have to keep records in order to deduct it on your Form 1040. Just give it away and forget it.
If the answer is no . . . Well, the answer is obvious.