See also one of the quirks of human psychology when money gets involved, i.e. we drop out of our social-reciprocal mindset and got straight to maximizing returns. Dan Ariely describes an interesting experiment in his books in which the idea was to examine price points and so on. They set up a table with little candies (Starburst, if I recall correctly) and set a price. They tried a variety of prices, and mostly it made sense; the lower the price, the more people bought. But they noticed a weird thing: when they dropped the price from one penny per candy to zero, i.e. take as many as you want free of charge, suddenly everyone stopped buying dozens of them and only took one or two. But a price tag of even a single penny sent people right back into mercenary mode, buying a hundred candies and emptying the bowl, etc. The theory is that free candies feel like a gift, and in a social/friendly situation, it's rude to take all of the thing offered to you, whereas if you find something for sale cheap, then there's no social obligation to be polite about it; you can rationalize that whoever is selling it could have asked for more if they wanted.
If you're a Big Name, you can get a couple thousand for a short story, easy. A few hundred dollars likely feels almost insultingly low. If you're giving it away, though, then you're doing a favor for another human, and your monkey brain calculates that differently than just monetary return, even though there's no reason for it to do so (since I doubt anyone at StarShipSofa is going to be grooming lice off of famous authors in return for their sociable gestures). (By which I mean that they don't really run in the same social circles, and therefore the likelihood of a reciprocal favor is low, but nonetheless the monkey brain just loooooves earning social credit by doing nice things for "tribe members.")