Code wanted ...google_conversion_value=???;

I am trying to use Google analytics to track the actual value of our orders. To do this i need to add a variable to the Google Tracking code. Preferably I would like to use a variable for the goods excluding any sales tax and shipping.

What would I use to complete the code:-

var google_conversion_value = 0;
if (???) {
  google_conversion_value = ???;
}