engine oil leak on 1999 Saab 9-5

I am leaking engine oil (approx 4"x6" spot) from the driver side, center part of the engine. I haven't noticed any other leaks. What could be leaking and how costly might it be?

Asked by for the 1999 Saab 9-5
there are many places for oil to emerge from an engine.
I'd suspect the oil filter's loose, which could be disaster if the gasket fails or the filter comes loose and all your oil comes out at high speed, on your rear tires and everything else under the car, including the extremely hot catalytic converter.
Get it checked out ASAP, it's a safety issue and a risk to your engine
If you want to find a specialist, here's a directory link for you: