Bank exchange charges

Hi, Can somebody please tell me how much banks charge for exchanging pesos into Dollars. For example if I've changing 850 pesos into dollars, how much will the banks charge for this transaction?

MrBart for that amount of pesos, just visit you nearest cambio place and you just buy the dollars straight up. No fees or anything just pay the dollar buy price.
But for larger amounts it is best to establish a relationship with your fave bank and get a better rate (in your favor) or ask sometimes the tellers are just a question away from helping you.
Thanks Grazie, I'm just thinking of paying my rent each month, as most of the rooms which I'm interested in list the cost in US dollars/month.