Yes, at $3.8M it is expensive, but it is a +90% sure kill; Mark 14s were expected to be launch in salvos and a 10% success rate more than acceptable according to :
http://www.hnsa.org/doc/subsinpacific.htm#pg9
"Number of Torpedoes Fired by U.S. Submarines
Total number fired = 14,748
Average number fired per attack = 3.586 Average number fired per ship sunk = 14,748 /1,392 = 10.59.
(8 in 1942; 11.7 in 1943; 10 in 1944) "
Something interesting I recently read: at the Java Sea battle, the Japanese launched 92 torpedos in one salvo and achieved 1 hit ! ... and that was in some ways successful since in the end they won the battle with fewer losses than if those torpedoes would not had been fired