Ok, the 3.1 engine would require 4 hours and an oil pan gasket that is $69.23. I cannot be certain of the exact hourly rate at the shop your are dealing with, but I would figure $100 per hour to be a good national average. So at this average rate, I would figure this repair would be $469.23 plus an oil change.
If this engine is the 3.8 model, you will need to figure for 4.2 hours of labor and $59.99 for the gasket. So this repair should be close to $479.99 plus the oil change. Feel free to contact me with any questions or problems! Good luck!