"15 Yuan. Thanks!"
I have the same question. But if i remember correctly from my lessons it should be correct as well.
Is 块 － the measure word for currency or really yuan. So，what does mean 元？ Can I say， for example， 一百块元？
Here 块 or 元 are both measure words, for money.
You can say 一百块钱, but not 一百块元.
I think 元 is the currency, whereas 块，毛，分 are the denominations. It is like in UK we have sterling as the currency, and we have denominations of £ (pounds) and p (pence). (We used to have £sd before 'decimalisation').