The percentages vary depending how anxious the agancy is to establish itself with your company. I've seen between 12.5 and 20% recently. There's a lot of agencies and the competition is fierce.
A good agency will thoroughly screen candidates, ability test, check references, etc before recommending them.
There is usually a clause that allows for a refund of some or all of the fee should the employee leave or not suit within an agreed period of time.
The above applies to permanent staff - agencies usually provide temps at an agreed hourly rate and pay the employee directly.