>Typically corruption undermines normal competition, reducing efficiencies.
...
>corruption increases the cost of doing business
these statements make sense and are true only than normal competition is a real alternative. Which it isn't in those corrupted countries.
Without normal competition as a feasible alternative, the things are the other way around. Corruption way of doing things is cheaper, while doing things "by the book" is much more expensive if at all possible (an attempt to do "by the book" is basically denying government people an income they are "entitled to" and they don't take it lightly and would make sure that it is less profitable for you :)
"On the macro level, corruption distorts market mechanisms, like fair competition...The World Economic Forum estimates that corruption increases the cost of doing business by up to 10% on average. "
As to whether ISIS oil is cheaper, I don't know about that, but if so, perhaps that could be because it was stolen from its rightful owners. I don't think you can use that to draw a conclusion that corruption causes costs to go down.