My father (in his mid 40s) recently took up trucking to help pay off credit card debt (my parents claim to be good with money, but I do not predict they'll pay off anything in their lifetimes barring winning a lottery or living much longer than average). He also owns an electrical/construction contracting business, so he only takes a maximum of one load a week (he usually does much less; more like one or two a month). He hauls steel, primarily from Tennessee and Arkansas to Texas (other trips come up, but I haven't heard of him accepting any). I think each trip is supposed to net him $1000, although there isn't a reliable schedule on when he gets paid--basically whenever the people in charge get to him in their list of priorities. (He also wound up working for a business in which one of his high school friends has a lot of status, which probably makes it easier for him to bug them about when he's getting paid--although at any given moment they seem to owe him around $5000, from what I hear.)

So just generalizing from the one datapoint, your $30-40k/year sounds about right.