As you can tell, a loaded question. Like mentioned, $500+ You will always find other things that need done or replaced. Where does it start and stop!
As you said....The Factory times are with "brand new" cars in the dealership and the Chilton times are those from Experienced mechanics in the field with cars with miles on them...GM made some of their gears with fiber on the sprocket teeth to run quieter and those would get brittle and fall off....into the oil pan.....into the pick-up screen, reducing or eliminating oil pressure....also check the crankshaft where the seal rides as the seal can wear a groove into it and leak oil.
