Science fiction magazines are notoriously low-paying. It's in part a tradition from the pulp era (when pay was as low as a quarter-cent per word) and in part due to the low percentage of ad content versus editorial content. All-fiction magazines (whether they are SF, mystery, weird tales, or general fiction) have always aimed to present as much fiction as possible, in order to pull newsstand sales and maintain subscriptions. If you published an all-fiction magazine with 40 pages of advertising and 50 pages of fiction, those 50 pages had better be by known writers--who, along with production, printing, and distribution would probably cost more per issue than the advertisers bring in.
What do they pay? Right now, Analog is at less than 10 cents per word. Which comes to $250 for an average short story. The most I've been paid by the magazine was 15 cents per word, well over a decade ago, when print magazines were still selling well.
Why write for low-paying magazines? Having written for both Analog and Asimov's SF (first story in 1978), and served as assistant editor for Baen's New Destinies in the early 1980s, I can answer that question.
Getting published in Analog, Asimov's, F&SF, the late Galaxy and other "pro" SF/F magazines yields all sorts of benefits beyond the money, if you're a social person. Free drinks and free dinners at cons, maybe romantic companionship at cons if that's what you seek, watching people who ignored you before at cons sidling up to get in on your conversation--and all sorts of other egoboo. (SF fan lingo for "ego boost.")
You'll never get those fringes with literary mags that pay 5 or 10 cents per word. (And if you're an editor--look out! It's a whole new level.)
Besides, where else are you going to get those weird (though professional) stories published? And then there's the fact that published short fiction gets you noticed and can help sell novels.
There's lots of comeradship on tap, too--again, if you're a social person. The Science Fiction & Fantasy Writers of America (SFWA--it was too hard to pronounce with effs) is a grand ol' club of well over 1,000 members that provides newsletter-type publications, selected author services, contact with other writers who can't stop, and venues for endless argument and other entertainment. Check it out!
Oh, yeah: Sometimes you get money for years after a story is published. My second short story in Asimov's was reprinted four times, earning twice as much in reprint as the original sale. And all I had ot do was sign the checks.
--Mike On the Way to the Web