IMO, it should be good for 300-400,000 miles. Bad fuel, water in your fuel, electrical shorts, etc., can kill the pump.Shouldn't the injection pump last the life of the car?
There is always a little bit of fuel in the pump so even if you run the fuel lines dry, the pump should stay lubricated.
The quantity adjuster, the top half of the pump, can break and if it does, it's worth it to get a new pump.
I don't know how much better a brand new pump would be and what increase in fuel economy you'll get, but like chitty said, it's not economical at all. That's like buying a new car every 100,000 miles to save fuel. It will be small and not worth it.