Fifteen years ago, when the rental market was tight, agents were asking up to the equivalent of two months' rent as their commission. (Ouch!) I have no idea what it's running currently. If you're moving for work, you may want to ask your employer whether they'll cover this cost.
(For what it's worth, I wasn't very happy with the agent my employer hired, who among other things didn't understand the terms they were hired under. My best leads actually turned out to be the "buy and sell" board in my office; a co-worker's father had a place for rent that was both larger and cheaper than most of what I'd looked at -- and, of course, no finder's fee. It was in an unfashionable area rather than being right on top of shopping and shows and high-speed public transit, but that didn't bother me.)