From Strategy Page:
"Naval mines achieved several striking successes during World War II. In the Pacific, naval mines proved more destructive to the Japanese war effort than the atom bombs. During a 10 week period between April and August 1945, 12,000 mines were delivered by American bombers. These destroyed 1,250,000 tons of Japanese shipping (670 ships hit, 431 destroyed). That's 18 mines for each ship hit. The Americans had air superiority, so losses during these 1,500 missions amounted to only 15 planes, most of them to accidents. Had these missions been flown against opposition, losses would have been between 30 and 60 aircraft, plus similar losses to their fighter escorts. But even those losses were, in wartime, a victory if you destroyed or damaged 670 enemy ships.
A conventional submarine campaign was also waged against Japanese shipping. Comparisons to the mine campaign are interesting. A hundred submarines were involved in a campaign that ran for 45 months from December, 1941 to August, 1945. Some 4.8 million tons of enemy shipping was sunk. For every US submarine sailor lost using submarine launched torpedoes, 560 tons of enemy ships were sunk. During the mine campaign, 3,500 tons were sunk for each US fatality. On a cost basis, the difference was equally stark. Counting the cost of lost mine laying aircraft (B- 29's at $500,000 each) or torpedo armed submarine ($5 million each), we find that each ton of sunk shipping cost six dollars when using mines and fifty-five dollars when using submarines. These data was classified as secret until the 1970s. It indicates that mines might have been more effective than torpedoes even if the mines were delivered by submarine."