I think its the water pump, I fill the coolant with water everyday and it is empty once I make it to my destination.
I know this isn't exactly what you asked but if you have the v6 engine changing the water pump on this car is a very easy job. There is a video on youtube that shows you exactly how to do it, just search water pump gm 3.1. Just be sure to get all of the old gasket off and use rtv on the new gasket. You could probably change it yourself for less than $50 and 1 hour if you can do basic car maintenance.
Call the shop where you are going to have the work done to get the cost as it varies by area and shop. You really should not be driving it if it's leaking that bad: You stand a chance of warping the head.
