It sounds like you're asking several questions at the same time.
In terms of the symptoms of failure, you don't give a lot of information about your specific car. However, some basics always apply: leaks (especially at the bottom of the timing belt cover) and basic failure to move the coolant (are you overheating this summer?).
In terms of cost, you're looking at a fairly labor-intensive replacement. Have you already replaced the timing belt? That is in the same vicinity as the water pump and, if the mechanic is already replacing the timing belt, that's a good time to take care of the water pump itself.
Finally, in terms of consequences, this part is in the category of things that can bring your trip to a halt. No water pump = no coolant = no driving.
Of course, water pumps generally don't fail catastrophically, exploding in a cloud of coolant vapor. That said, it's going to go eventually. If you're lucky it will last for a while. Time to start setting a little cash aside for the eventual replacement?