Even US$13 or €8 is too much. We have Apple to thank for this, who have once again implemented a completely stupid idea in order to be able to sell their AirPods nonsense and the others then naturally imitate them. It didn't make smartphones any cheaper, but now everyone either has to buy new wireless headphones or buy and use some stupid adapter. Rationalising a headphone jack out of a smartphone is tantamount to a tyranny.
The biggest impertinence, however, is that no such adapter is included as standard with OnePlus devices. When you rationalise away something that is already established...