As a busy freelance writer, you should be able to use your time and resources more efficiently. Here's what you can do to make more money from your article:
Reslant It
You can write another feature article on the same topic but on a different angle or focus. For example, if you've written an article about being a vegan in general, you might want to consider writing another article focusing on vegans and pregnancy and sell it to another health magazine.
About two years ago, I wrote an article about organic farming and organic diet. The article was featured as a cover story in a local magazine. I was paid more than the regular industry rate for a 1200-word article. Not bad. But when I did the math, it turned out that I'd spent almost half of the article fee for food and transportation (I went to the countryside to interview an organic farm owner). And then there was the purchase of AA batteries for my voice recorder, Internet and printing cost for research, cell phone calls, etc. In my effort to offset things, I decided to write another article on organic diet, this time focusing on organic food for babies. I pitched the article to the editor of a health magazine I used to write for and immediately got a contract. I was paid again for that article. I was satisfied.
Rewrite It
You can lengthen, shorten, simplify or make your article more comprehensive. For example, you can write an article on Mad Cow Disease that's laden with technical terms and pitch it to a magazine focusing on animal farming. Then you can try rewriting it in simpler language and have it published in a science magazine read by high school students. To make things easier for you, keep in mind your audience and the level of language they use.
Resell It
In case you retain all rights to your published article, resell it. Some magazine contracts allow their writers to resell their article after a period of time. In my previous blogging experience, I was allowed to republish my article one month after it was posted on the blog.
If you're unsure about reselling your article, consult the editor of the magazine that first published it.