It is hard to say how long a radome will last. It could be years or it could be days. A bird strike could do alot of damage, a lighting strike could put a pinhole in the radome. Day to day wear and tear can cause the paint to chip, erode, or the fiberglass to delaminate.
As part of the pilots or mechanics walk-around, the radome is allways carefully looked at. As mechanics, we don't repair the radome in the field. We replace it and it will be sent to a repair shop and they will determine if it is repairable.
As for the cost, I really don't know, but I am sure it is expensive.