For one single family home, 8-10% is the average and of course, everything is negotiable!
The main thing is to ask them proper questions on how they will be managing your property. Ask what services are covered in their management fees. How do they manage evictions & cost of eviction. If they fill a vacancy, do they give you at least a 6 month guarantee? What do they charge to fill a vacancy? What do they look at in evaluating a tenant?
These are just some basic questions. Remember, that you are interviewing them!
That is a decent rate. I've understand some start at 10% and I've even heard of and know some charging 15%. Property Management agencies are worth every dime that you pay them - especially for piece of mind. Get a list of all the services they intend to provide for you and at that rate per month make sure that you are hands off of everything other than making sure they are performing for you.