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This ranges from 1 Cent per day to $1000 per day.
As with the normal distribution, the extremes are very unlikely, but 30 Cent to $30 per day covers maybe 99.9% of the chances.
Of course, you might get a higher or lower eCPM.
500 visitors a day is a fairly small number, so it's easy to get some wild variations, especially if it's narrow in focus - you might have some days were advertisers are wanting to spend extra, and so your earnings go up. You might also have days where something you have on your site gets linked from somewhere and you get an influx of traffic.
I think some of it depends on how people are finding your site, what they are looking for when they go to your site, and how often your site is updated - i.e. are you bringing in lots of new traffic or is this returning visitors.