Don't start to think about money first. What subject you may have a lot of knowledge? Then start with it first. Money comes later..
Blog on a subject you have some expertise in...I would advise you to try a television show of which you are fond. I did it and I have a marginally successful blog that has had as many as 45000 'uniques' on some days. I don't really do much at all in terms of promoting the blog; all I've done is blogged on a show no one else has and Google and word of mouth brought people to the site. After a while I encouraged readers to write and now there are four of us, five if you include another reader who makes videos related to the show.
Well, I wasn't thinking about the money. Don't know where you got that. But, before I started any of my sites, I did not even know you could make money, until a good 2-3 months later.
you should blog about what you like, not try to cater to anyone else. You will find it much easier to update it when its about something you enjoy rather than something you don't, and chances are there are more folks who like the same things you do.