It's not as much as it used to be. Go to http://truecar.com
and find out invoice.
I think it's around 5% difference.
So good deal is around 9-12% off list (but don't take my word for it, do your homework)
To get above 4% off invoice probably requires group buy deal. Unless it's old 2014 stock.
I like using cars.com to see what dealers around the country are asking.
It gives you a good idea of what kind of deal you could get.
I have not experienced hidden costs beyond what has shown on cars.com
It does not include tax, does include destination,does not include "document fees", may or may not include current incentives but the lowest priced ones usually do include the current incentives so don't expect further discounts on the bargains you see on line.