This will depend on your portfolio, experience, reputation, motives and ambition - and level of financial need, I suppose!
Such payment will always be by negotiation. One word here I would give is never be afraid to negotiate. As long as your terms are reasonable they will be considered as seriously as they are meant.
In real life, I would say never, ever, ever sell all your rights. However, in the spirit of the question you will be offered (in the real world) anything from 0.01c a word up to 0.25c a word depending on the work, your 'status' and the publisher. Either that or a round figure for the article completed and submitted on time. For a piece of 1000 words that could be anything from $25 up to $500, even more if you can swing it or have very specialist knowledge. Again it depends on so many factors it is very hard to generalize.
Personally I wouldn't undertake a professional engagement of 1000 words for less than $150 - as a working but 'unrecognized' writer.
For a genre fiction 'zine that I liked and wasn't making anyone rich I might do it for less.
For a cause I believe in, free.
The only way to get a good yardstick if you feel you need one is to join one of the many professional writers' associations or unions - although you might be depressed to work for their 'minimum agreed rates.'
I think you sell a piece on the basis of a compromise between what you need, what you think it's worth and what you can get. If you are happy and your publisher is happy then that is that.
Sorry not to be clearer but it is such a fluid, flexible, open market it really is impossible to give a standard rate - even more so on line than in print.
I hope that is some sort of help.