how much?

Difficult to say. Most garages charge a minimum half hour labour, even if they spot the problem straight away. I wouldn't expect it to take much more than one hour to find, even if they have to pressure test it or remove covers to check the water pump.
Cost of repair could vary enormously, as the cause could be anything from a loose hose clip to a failed cylinder head gasket.
Roughly how much water are you losing per hundred miles?
Can you see any water dripping?
Is the car running OK?

barry
May 2009