Sir I have no idea about the cost but I know its not a very complex job to replace if the mechanic knows what he is doing.
Also always fill the torque converter with ATF before installing and Replace the seal with a new one.
But as some other members have suggested at 140k mileage it is not usual to have a problem with your transmission unless you were abusing the car badly.
Better visit a mechanic and get it scanned, most likely it will throw a code.