Change the currency

I would like to change the currency of my plan payment from  American DOLLAR  to Brazilian REAL. However, before changing it ,  I would like to know how much I will pay for the same plan in REAL,  in case I decide to change it. Unfortunately, it hasn't been possible to find this information anywhere, before I decide it for good. To make things worse, once I change the currency, I won't be able to return to the previous one anymore!