For minesweepers (and air defense missiles too for that matter) the cost-benefit isn't just "cost of minesweeper over cost of mine", it's "cost of minesweeper over cost of mine plus cost of ships that run into mines". Also, minesweepers have an advantage in terms of concentration: you have to mine an entire area for mines to be fully effective, but you only need to proof one mine-free lane for the minesweepers to get their cargo through. (That being said, the best use of minefields is to canalize targets so they show up in areas you're expecting and are prepared for, which can shift the balance back the other way too.)