If you're dedicated and write unique articles about topics that others on the internet are searching for you can make quite a bit of money.
Here's the list of success stories http://hubpages.com/success/ It's very inspiring and shows the broad range of earnings. Some earn a lot with only a couple hundred hubs, some are earning the same amount by having a thousand hubs. It all depends on what you're writing.
Personally, I make about $30 a month, but I've only been actively hubbing for a few months now.
Keep in mind that some hubs can take months if not years before they really hit their stride (I had one hub that didn't get popular until 2 years after it was published, now I get several hundred views a day). Each one builds and makes you money. Each hub isn't just adding earnings, it's multiplying, especially if you write several hubs around the same topic.
The learning center http://hubpages.com/learningcenter/contents is a great place to learn everything you need to know about producing great hubs.