Calculate the potential CTR (factor 1) by investigating the eventual traffic (factor 2) and then use the average CPC (factor 3) for your keywords to find out the monthly revenue.
I think its too hard to get that much from blog. but I too belive that you should work with high paying keywords.
There are a bunch of people who make $200 a month. It takes a good niche with good unique ( not spun) content and good keywords.
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nothing is impossible u can earn 200 or 2000$ it depends on ur efforts, one day u will get 200 per month and u will start dreaming 400$ per month , it is human nature. write quality content and build traffic = adsense secret.