It also depends on whether you want an agent just to find a tenant for you, or whether you want ongoing management services.
I can tell you, anecdotally, how much I'm charged by a Realtor to find tenants for me. The one I use charges one month's rent as a commission. That's just for finding a tenant, not for management. However, that's just my experience, and with only one agent. Best thing to do would be to call a number in your area. After 3 or so, you should have a pretty good idea how much it'll cost you.
Hope that helps.