The U.S. military has begun using much cheaper weapons systems to defend against Iranian drones, forcing them to use more exquisite unmanned systems, according to U.S. Central Command Commander Adm. Brad Cooper.

U.S. troops had relied on expensive air defense systems, each missile with a cost upwards of multiple millions of dollars a piece, to stop incoming drones that have cost as little as tens of thousands of dollars. The math wasn’t in the U.S.’s favor, but Cooper said on Thursday that the calculations have begun to change.