Different organizations offer suggested pay rates for different writing work.
Although PWAC is Canadian, their list gives you an idea of the range. http://www.writers.ca/whattopay.htm
Also, Writing-World website has a good article 'How Much Should a Freelancer Charge' - http://www.writing-world.com/rights/fees.shtml
I'd always suggest an hourly rate rather than per issue as it will ensure you get paid for the time put in making changes suggested by the newsletter owner and accomodating any of the variables that you can't always plan on.
On the other hand, if it's a regular gig, once you've done two or three issues you have an overview of what each one takes and might come up with a price that is an average.
I do two newsletters right now. One's monthly and I charge a flat $450 an issue; the other's quarterly with more pages, I write more of the copy and use a more sohisticated layout, which usually comes in just under $1,000. My hourly rate rages from $25 - $65 depending on the who the client is and how much original research/writing/design is required.
Hope this helps.
LoisP
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Lois Peterson
www.lpwordsolutions.com
Author of 101 - and More - Writing Exercises to Get You Started & Keep You Going
www.mettapublications.com
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Lois J. Peterson, writer ~ editor ~ instructor
www.lpwordsolutions.com