I did it on my wife's Roadster a couple of years ago.
I am not sure what transmission is in your car and I bet it is different than the one in our older Roadster so parts will be different.
I used Amsoil Universal ATF. From what I remember, six quarts of fluid was about $50.00, new filter and pan gasket was about $65.00.
I changed it at 50K and the fluid was dark black and very, very thin. After the the change the transmission was quieter, smoother and shifted about 200RPM lower than before. That was 20K miles ago.
Link to what I did:
http://www.shipkiller.com/Autoxmission_fluid.html