I've received estimates of $500-$650 to replace an oil pan gasket on a '99 gran marquis. Why does it cost so much?

Are you kidding? When replacing a pan gasket they have to remove all the fluid. But it should only cost that of a standard trans fluid service. Which at most transmission shops shouldnt cost any more than $60 tops. If you are so inclined, most oil shops will let you bring your own oils filters and gaskets if you have a preference on a brand they don't carry. But $500-$650???? No. Can you say RIP OFF? Get out The yellow pages and make some more calls.

Are you getting these estimates from dealerships or transmission shops? I'm familiar to an extent with these cars but without looking for myself to see if there is a cross member blocking the pan, they are relatively simple for a person with basic mechanical skills to do it yourself. Usually there are about 12 screws to remove. You could save yourself a bundle by doing it yourself. when putting it back together use a small amount of gasket sealer to hold the gasket in place. Get a large oil pan. Let one side of the pan down and let it drain off first. I personally can't afford a mechanic to work on my truck so I've had to learn how to work on my own vehicles. It's really not hard. All it takes is the right tools and some common sense. Oh. If you still have the owners manual it will tell you pretty much everything about your car and when to service it and how much fluid it takes. If you decide to do it yourself be sure not to over tighten the bolts, you will pinch the gasket. Once the bolt is seated give it another 1/4 to 1/2 turn. drive till its warmed up then inspect for leaks. let me know how it goes.