EDIT: Actually I don't really know what I make on average per jump. 30₢ is merely the (conservative!) estimation for milk-runs. But nowadays I am more a seller than a buyer, if you know what I mean. Or, to put it differently: I tend to fill my cargo hold "on the fly" rather than shipping commodities from outside the system myself. You could also say that I am employing sub-contractors for the tedious work of witchjumping cargo, although they are usually not aware that they are my sub-contractors, until I take over their cargo. I think you get the idea what I'm talking about.
So, for me the selling price equals the net profit, but of course I also get to sell a lot of low-price commodities this way. Still I think that an average of 30₢ is not too far from the truth.