How much does nem charge extra to buy nem on its site?
Could you clarify your question? As I know you can’t buy XEM on nem.io site. Only some exchanges, changelly and so on.
NEM doesn’t charge anyone a premium to buy XEM. XEM is traded on the open market.
Perhaps you are referring to the Changelly widget in Nanowallet?
Changelly seems to charge a premium for credit card transactions.