Good content, good SEO and some time and it's easy. We've had .info domains that made upper $xxxx in three months (event) and have had monthly adsense revenue of over $500 for a couple years.
Just start with the intent of building the best site you can and they will come. Of course, topics and keywords need to be popular and you may have to work at it for a few years.
I have a rule of thumb with adsense - 1000 impressions = $1. This may seem like a low figure and can easily be doubled for sites less than 1000 pageviews a day or tenfold for niche keywords.
The thing is that the huge keywords have smaller ppc. I have found that once you start getting 100,000 daily pageviews your basically looking at $100 a day on average.
Do the calculations. You want to earn $500. That's about $17. a day or 17,000 impressions daily. Now the question is, how many impression do you get per visitor. To simplify the math let's say an average visitor gets 4.25 impressions. You then need 4000 daily visitors or 120,000 monthly. If you could get 1% of the traffic from a keyword searched 12,000,000 times a month then your in business.
Work hard to develop and promote those domains and in five years you can quit your job and do it full time. Hint - build sites that don't have to be updated on a daily or weekly basis such as math/science, history, geography, biography. Example: I'm now working on a site for guys of any age to meet a girl for a long lasting relationship. This will change with time but basically can remain for relevant for decades.
