Average income from Google AdSense varies across websites based on the traffic they get, the topic on which they are writing, the competition for the advertising, the target audience, as well as on the time of the year. Given this wide range of factors, it is difficult to model or track your site's results against any given website, or even across a range of your own websites.
While there are Google websites that generate north of $40,000/month and several others that generate $10,000/month, achieving those goals requires concentrated efforts with websites filled with content with over 1000 pages built over a period of years. Moreover, for a single website to generate the type of traffic needed to get $10,000/month at an average payout of $.25/click, and say a click rate of 5% (all hypothetical numbers) is 800,000/month, i.e. 67000/day. It is worth noting that several well-established websites do not have a traffic greater than 3000/day, which would imply a Google AdSense income average of $1100/month. For this reason, several webmasters develop multiple websites so that they can generate a combined income across their sites in the range of $3000-$5000/month.
A well-maintained site with good quality content will remain valuable property on the Internet for a long time to come, so your overall efforts in building such a site will pay off over the long run.