well the market's competitive, and so it would be really difficult to have a short story sell even once a month for that amount. But I seriously discourage anyone from ghostwriting short stories. That is a lot of credit to throw away.
The short story market is competitive. Extremely competitive. Far, far more competitive than the novel market. But most of the completion takes place at the bottom, not at the top. There is competition at the top, but primarily for individual magazines, not all magazines. Experienced short story writers who are reasonably prolific should be able to sell a couple of short stories per month to pretty good magazines.
The competition for new writers is unbelievably keep. There are millions who try writing short stories, but for new writers, good isn't good enough. In order to sell a short story to a really good magazines, a new writer must write a story that's better than what the editor receives from top writers that month. Not as good as, but
better in some real way.
It's funny, but I don't think I've ever sold a short story in the range of five to seven hundred dollars. I've sold a lot of short stories in the range of three hundred to about four-fifty, and I've sold quite few short stories for eight to nine hundred, and a quite a few for a thousand plus, but never in that one range. I have no idea why that is.
Anyway, ghosting doesn't automatically mean giving up credit. I've ghosted novels, and never had to give up credit with the people who matter. My agent knows which novels I've ghosted, and the publisher knows. Beyond this, who cares.
I doubt it would work that way for short stories, but if you aren't good enough to sell to magazines, why not ghost? No credit plus six hundred dollars is better than no credit and no money.