It's all about how low can you buy and how high can you sell.
The expensive commodities usually have higher return profit per ton.
Say there is a system where you can buy Gold for 8800 credits per ton and next door there is a system where you can sell gold for 10540 credits per ton. Now that is roughly 1700 credits per ton you just made by ferrying Gold from one station to another. However if you buy gold at a 9999 price and sell at 10540 you will only make 541 credits per ton. It's decent money, but I would suggest looking for another commodity with higher profit margin.
Some of most profitable commodities are: Gold, Beryllium, Superconductors and Progenitor Cells. But when trading you must always look for something that is selling significantly bellow galactic average and then try to find a place that would buy at above galactic average.