Not too low, because the US and British oil companies have to make plenty of profit, and these countries also have to earn profits which they can then send back to their masters in London and New York. So, not too low. But also not too high, because you want to keep trade efficient. I'm not even mentioning so-called externalities, like pollution and so on. If the real costs of trade were calculated, the apparent efficiency of trade would certainly drop substantially.
Nobody knows how much.