How Much Under Invoice for a 2009 ?
^ check that thread out for 2009 models. You should really be getting around $2k below invoice on 09s.
As far as 2010 models go, there are a few threads on the boards, but they aren't really good threads specific to touring w/ sport package. Regardless, right now they are not moving very many Z's (a lot has to do with the fact that its still winter). I would say a fair price would be $500 below invoice, but recently people have been doing even better than that on 2010 models.