Like others say, it varies. A lot. Based on things that you may have no control over.
I operate out of the Chicago area, for a company that typically has us van drivers drive out loaded/come back dead-head. Loaded miles paid, dead-head on your own hook. This only works because I'm in the Chicago area, if you're not in a huge base market like Chicago or New York this doesn't work well. For most drivers, you're probably going point-to-point, stopping at or near the last stop waiting for the next load. So, figure in how you're going to occupy time near Champaign, Illinois if you live in Kansas City, MO and you realise it's going to be a while (like a couple of months) before you get home again.
You've got to eat, in winter you've got to keep warm, which means either burning gas/diesel or finding a place to stay, both are expensive, and one way or another you've got to find ways to fill down-time, since it's guaranteed that you will have down-time.
So, how much you can make depends-- on how much you're getting paid minus your expenses, and if you're getting paid per mile, how many of those miles get paid. Something less than half of my total miles are unpaid dead-head (back-haul freight and a couple of runs where I didn't quite make it home before getting called on the next run cut into the dead-head), and remember that those dead-head miles still burn fuel which comes out of your total pay. Maintenance and break-downs figure in too, especially for those of us who thought we'd make big bucks being owner-operators of our vehicles.
Think that last item isn't a worry? OK--- last July I broke down in Tuscola, Illinois. I had a Sprinter, and the turbocharger broke. Before everything was said and done, it wiped me out (there was more than the turbo, that only started an expensive chain-reaction) and it's only because I have friends that I was able to get back on my feet. Otherwise, I'd probably be living on the street right now. No joke. Right now I'm driving a Chevy van which is older than dust, but it's still got some miles left if I can keep the maintenance up.
Don't be scared away, but do be aware that this business isn't a gold mine. Especially today when everybody is cutting expenses at our cost.