Damascus steel knives are quite expensive. You’ll be looking at $100 and up—and that’s just for the blade. A custom knife can cost you upwards of $1,000, but it will last a lifetime.
As for how much you should spend on one yourself, it depends on what kind of use it gets. If you’re using your Damascus blade as an occasional carving or slicing tool for special occasions (or if it’s strictly for decoration), then you can get away with spending less than if you were planning to use it daily in your kitchen or outdoors.