I have been building websites for over a year, total novice when i started, have over 30 sites now and still building and I have not made £100 in any one month yet, i only do this part time though,
There are many hidden costs, ie domain name, hosting, software, design, writting and time.
So my advice build each website about something that your really interested in find something that works for you, I have a set way of building my sites, that works for me. Everyone one needs to do what they are good at and out -source the rest.
Also build it up slowly, if you build good websites with good content, the search engines will rank you, then maybe add some ads and products which can be a very good source of income. Hope I have been a help.