Not much is the answer, certainly not enough to support yourself, fund your retirement, etc. There was a point when I first joined where some hubbers were making $1,000 - $3,000 or so a month and this information was freely available through HP. Others state these hubbers are no longer making those amounts, their income has dropped. With various changes outside of HP, many hubbers state their income has dropped. A lot has changed in the 2 years I've been here. But now, things are different and as for the real data you seek, we are actually not encouraged to post our actual earnings.
What I can say that yes, my earnings have grown and can pay a cable/web/phone bill or car insurance bill after two years of being active here. If those are your only expenses, great. Otherwise, don't hold your breath or count on growing your efforts to a single, life sustaining source of income. I enjoy my interactions with others here, reading others hubs and that is why I like HubPages and have stuck around.