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[edited by: eljefe3 at 6:47 am (utc) on May 23, 2006]
500 uniques per month is really small in terms of general traffic. Saying that, a highly targetted site with a good conversion rate could charge a decent price for advertising.
Check out your competitors. Do any of them post a rate card or advertising info?
If you can't find any of that information, just propose your fee.
I'd collect the money in advance if possible, until you've built a trusting relationship with the advertiser.
Hope that helps.