By default the Web Store reflects the price in USD. When you select an item to purchase it will first ask you to select a payment method, once you've done this scroll down to Green Text that shows your Total Price. 
You can tap on this Price to see your subtotal before and after conversion.

PLEASE NOTE: If you have saved your card on file, selecting this card again will automatically complete the purchase. 

In this case to see the breakdown, you'll need to select "Choose Another Payment Method", then select "Credit/Debit Cards" and scroll down to see the Total Price!