2004 Hyundai Sonata tranny slipping and spilling

A couple of weeks ago I noticed a spill of tranny fluid in my garage. Also noticed that it would slip slightly in low gears. Engine and tach will rev but no real response in speed. Took it to a mechanic and he said that the leak was coming from a gasket next to the torque converter_ 5$ part and 700 in labor to replace. He also told me that there is no telling if there is any permanent damage until he goes in there and looks more intensely. Also told me that there is transmission fluid coming out where I should be filling it in the engine like a geyser. I need to know what is going on with my car and whether or not my tranny will blow any time soon. Please help