Here’s a screenshot of Google’s adsense report for the last 7 days. 
It basically says that for 501 page impressions, I have had 11 clickthroughs (Note that Google ads dont appear on all my page impressions). That’s around 2.2% of page impressions that get a clickthrough. From my understanding the average is around 1.2%, so one can conclude that the adds are reasonably well targetted. Either that or my mum is clicking on them to help.
The CPM number is the importnat one. It basically states what you are earning for every 1000 page displayed. Here it is $2.75 … that means that if I had 1,000,000 page impressions, I could expect a revenue of around USD2,750.00.
But more importantly, I can now work out how many page impressions I would need to cover the costs of the server. The server costs around USD40 per month. To cover that cost I would need 40 / 2.75 * 1000 = 14,545 page impressions per month. That’s around 485 impressions per day.
I’m currently averaging around 200 page impressions per day… so I am a little way away from that right now. But doubling the traffic I think is not that far away … so I think I should soon be able to cover my costs.