You can check to make sure you're dealing with a licensed entity here:
They should be licensed under real estate.
I would have two concerns if it's monthly. First, are you dealing through a legitimate entity or is it some sort of Craigslist type scam? Second, is the administrative fee locked for the same duration as the lease? You don't want to be locked into a one year lease at $1,800 with a $200 admin fee, and then have the admin free raised to $500 six months into the lease.
Hopefully whether it's monthly or one-time will be clarified, and if monthly, someone can shed some light on this practice, because I'm coming up with nothing.