Coolant leaks very fast.

My 2001 Aztek has 120K miles on it. I refilled the coolant, but it all leaked after a 40 mile ride. A mechanic told me the gasket at the top of the engine was bad, and it would take about 1000 dollars to fix. I am trying to get some perspective about it before I take any action: how serious is the problem? Is it as serious as driving with bad brakes, or it is something I can neglect for a while? Forgive my ignorance about cars.