2000 Pontiac Grand Am Replace gasket cost

I was recently told that I needed to replace the upper and lower intake manifold gasket because there is a pressure leak and you can here a hissing noise when car is off. I have gotten estimates ranging from $300.00 (240.00 labor and 60.00 parts) to $900.00 (700.00 labor and 200.00 parts) With such a huge difference I don't know who to trust. What should be the range lowest to highest I should expect to pay for such a job. Thank you Very Much!