The sole reason one would want to place ads on their website is, of course, to earn money. But how much money does that mean more precisely?
An average website can bring in anywhere between $0 to ~$3000 each month. It is a big spread, yes, but the income is strictly related by how much traffic your website has each month. The other factor that influence the actual revenue is RPM, which is calculated based on a combination of your traffic, CTR (click-through-rate), and CPC (cost per click).
Let`s assume you have a RPM around $3 and you get 1000 visits per day, the average earning will be $3/day which gets to around $90/month. Pretty low and maybe nowhere near what you would have thought of, but there is still hope.
Increasing traffic is one way of boosting revenues. Of course no one will visit a website without any interest, so content-writing is your friend. Always bring fresh new readable content to your website and visitors will sure to follow. Writing a nice blog post every other day for a year will grow your article count by ~300, which means more traffic. Taking the above example with a RPM of $3, which is quite low, it can go as high as $50 for high-tier websites. Increasing the daily traffic from 1000 to 10.000 will mean your website can bring ~$30 each day and ~$900 each month. It already starts to look better.
Another way of increasing earnings is trying to increase your RPM. To better explain this I am going to use an example.
A general blog, even with thousands of articles, will have a general lower RPM than a niche tech blog for example. The ad publisher has to pay more to publish his electronic gadget ad ( as an example ) on the tech niched blog, than on the more general blog. This is determined by lots of algorithms analysing what type of traffic goes where. So having a more niched website will bring up the RPM and having an RPM near $10, that would mean ~$100/day at only 1000 visitors each day.
Increasing both traffic and RPM would mean the best possible way to gain more. However do not concentrate solely on doing this. Traffic will come naturally if you keep adding fresh content and RPM will go up if you keep adding niched and quality content.
Never try to create a new website for the sole purpose of generating income through AdSense. It will never work. It may not even get approved by Google in the first place.
If you do everything right, your website may get up there in the top 1% of websites that can generate enough income to make a living out of them.
Most of the websites remain, though, in the rest of the 99% that make a few $ each month or even less, not enough for anything.