Back in the 1800's, when he was renting properties on LawnGuyland, someone asked him how much he would charge in order to faciliate a rental. Charles was hard-of-hearing and thought they asked him his surname, so he quickly replied "Onemonth", and so began the legacy of 1-month-rent as the broker's fee.
Aren't you glad you asked?
Just so you know....and this may not be feasible in your area.....at times I have seen the landlord offer to split the comission with the tenant as an incentive. My friend is a landlord, and she recently did that in order to attract a tenant after her condo had been on the market for a couple of months with no offers. It doesn't hurt to ask.......the landlord can always say no.