It should be 35 euros return for walk up tickets (you will be buying two singles; return tickets are twice the price of singles and there aren't really period returns). There is a small charge for buying from a manned ticket office as opposed to a machine, and in the (two or three?) stations that accept them, a charge for using non-Dutch credit cards. Amsterdam Centraal is one of the ones that accepts them. These charges are both somewhere around 0.50/1.50 euro I believe.
The NS website should help you plan it
here. You will be paying the full fare, unless you have bought some monthly/annual discount on your registered OV-chipkaart (smart travel card). You can also purchase a ticket in advance from this site
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Ah, just checked and you will need a Dutch bank account to purchase from the NS website.
You could buy from Rail Europe but they'd charge (£8) for postage and there is no discount for buying in advance on Dutch railways, so if you have the time it'd be cheaper to buy on the day.
All ticket machines have an English option and the ticket staff speak English so that shouldn't be an issue. Otherwise, maybe someone else knows another website?