Picking a topic for website
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Estimating how much advertisers are willing to pay
Different types of advertisers are willing to pay different amounts of money for different types of content. Therefore you should spend some time researching to find a topic where you have higher paying advertisers. Just recently I came accross a blog, where the author was discussing his Adsense earnings. He was earning about 20 times less from his blog with 3 times more traffic than myself from one of my sites - that is 60 times less earnings from each visitor! Content type is not the only factor determining your earnings, but one of the main.
For estimating how much advertisers are willing to pay for ads on your site there are a number of ways. The most obvious, if you have some potential advertisers in mind, you can contact them (although they are not likely to discuss anything with you until you already have a site with established traffic). You can check what banner networks are paying.
If you decide to concentrate on pay-per-click programs, which have been working out the best for me, there are some tools you can use to figure out per click earnings that these programs are offering to publishers. If we take Google's Adwords program (the Adsense counterpart) as example, you can open an advertisers's account, which offers advertisers various tools to estimate the amount of money an advertiser should expect to pay for a click and a total amount of clicks that can be expected. The Adwords traffic estimator tool in particular can help figure out a rough idea about average click cost for a certain keyword. As a publisher you should expect to get a portion of that cost. You won't get exact amount but you can estimate relative 'popularity' of a keyword.
Keep in mind that the above step is quite important and you should spend some time researching and experimenting as it will play a key role in how much income you will be able to generate.