As per what Cl4life said most people need multiple blogs. I know one guy which I talk to regularly has approx 100 adult blogs and combined they bring in around $100 a week. He beleives that small gains on each blog keep his average earnings pretty stable. If one blog does bad one week another one usually picks up. He commented he wanted to make over 1000 sites.
On the other hand I know another guy who runs around 3 blogs and can make $50 a week but he has to manually promote, post, etc. Its higher quality blogs and the earnings are there but he wastes an hour each day keeping them up to standard.
Which would you prefer ? An automated 100 blogs with no work for $100 a week or 1 hour a day for $50 a week.
I prefer the 100 blogs method as its less work in the long run.
If I could find an automated method to have a high quality blog that earns approx $2 per day per blog I would take it but all the automated blogs I have seen are lucky to pull in $0.20 a day per blog hence their creators make hundreds of them.
Take the first guy. He earns roughly $1 per blog per week and if he ever gets to 1000 blogs he will be earning $1000 per week. He had set up the 100 blogs over 3 months so I don't doubt he will get to 100 blogs if he can find the content and niches to keep putting up blogs.
My experience is that I have a few blogs (approx 20) and they make roughly 3c per post. I have semi-automated them and I review the content every few days. I post roughly 10 times each day per blog. So as you can work out 20 blogs * 10 posts * 3c per post = $6.00 per day.
Its cump change to many but I know I can slowly upscale this method with more blogs and more posts.