Forrest J. Ackerman wrote not just the shortest science fiction story ever, but what may be the shortest SF story possible (if you ignore the title). The title is "Cosmic Report Card: Earth." The story is:


For this he was paid $100 for the initial sale, as well as for four more sales, so he was paid (he estimates) at a nominal $500 per word per sale, or $2500 per word for all five. Not bad for a night's work, especially in 1973.

Source: Mimosa 16, pp. 4-6