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Do you have a certain method that you use to decide?

I offer all three, you don't have to decide on just one. Price accordingly, for example, with use you can reuse the article other places/sell it multiple times. Full rights sell for higher dollars initially, but you can't do anything with the article after that, it's like the person who bought it wrote it. Look around at articles and you can see how other authors price. There is a wide variety out there.

I was just about to say what Elizabeth said. Offer all three. As for pricing, if for example you ask $40 for full rights, then perhaps ask $30 for unique and $20 for usage. Again, good advice to take a look at what others are asking for comparable length work, and use that as a guide.