Replacement tailcaps are $12:
http://www.fenix-store.com/product_i...roducts_id=219
or you can try taking the bad one apart and see if you can figure out what's wrong. Maybe a cracked circuit board or the spring broke off. Sometimes, the retaining ring for the tail switch just loosens. A pair of needlenose pliers in the 2 ring holes can be used to take out the switch, if you don't have ring pliers.