When I worked at a dealership the employee discount was $200 over the invoice of the car minus any available rebates. So you were getting the vehicle for anywhere from $800-$4,000 below invoice costs.
I haven't checked in on how they are coming up with the Employee Discount pricing for cars these days, but hopefully people can still spot a good deal when they see one.
And if you are ever skeptical about how good of a deal you are getting on a new car, you can always insist on seeing the actual invoice of the car. They won't want to do it at first, but when you push for it and let them know you have no intentions of staying if they don't cooperate with you they will usually bring it out.