I had one go on my old 216. Fortunately it was only a steady seepage and not a major leak so only needed the gasket replacing and oil and filter change. Still cost over £250 though!
If they go in a big way the head may need skimming (the water leaking through under pressure can scour the metal or if the engine overheats, the head may warp slightly). Both oil and water systems will need flushing too. This all usually takes the cost top side of £600.
Sudden water loss and overheating are the main symptoms of a major head gasket failure. If you have a steady loss (you have to top up the system regularly) it may be the head gasket also.
Check the water in the header tank for an oily film and check the oil dipstick and oil filler cap for a grey emulsified deposit. If the head gasket has gone you will usually find both. :evil: