Minimum impressions
Here are some figures that have been floating around in my mind that I thought I would share.
A website would need to generate 500 thousand impressions per month to receive an income of $5000 monthly. If you take 30% off (for taxes that amounts to about $3500. This would provide a modest monthly income for a one person show. The site would be about the popularity of cooltools or treonauts.
SITE - IMPRESSIONS laughingsquid[fm] - 390,000 cooltools[fm] - 480,000 treonauts[fm] - 540,000 askaninja[fm] - 780,000 43folders[fm] - 900,000 boingboing[fm] - 7,380,000
It's difficult to compare impressions since many websites don't offer that sort of information but you can use www.compete.com to get an idea of how the site ranks compared to others. Choose a couple of the sites you know the impressions for (laughingsquid, treonauts, askaninja as a baseline) as a baseline and compare the graphs to get a rough estimate. For example, tribe.net seems to far exceed the traffic of laughingsquid whereas tinynibbles.com seems to come pretty close to treeonauts.com. I've compiled an excel spreadsheet of sample impressions if you're interested in getting a better idea of what the numbers mean.
Here's how the numbers work out. If you were a website that charged $5 CPM (a modest figure, Gawker media charges between $6 and $10), you could sell 200 thousand impressions to 5 customers and earn $5000 monthly. This means your site would need to generate 1 million (one thousand thousand) impressions per month. You can double up the ads on a page and reduce this number to 500 thousand.
($5 per thousand impressions) * (200 thousand impressions) * (5 customers) = $5000 monthly (200 thousand impressions) * (5 customers) = 1000 thousand impressions