I think we know that the problem isn't detecting the enemy UAVs but affordably disabling them. My suggestion, just off the top of my head, would be to equip your own drones with the ability to spray a web of synthetic silk-like material which expands into a web and which becomes wrapped around the rotors. The material itself would not be strong enough to cause engine seizure, but would instead be designed to act as an electrical inducer which converts electrical energy which leaks from the motors (which are cheap and leak a lot of energy as magnetism) and turns them into current, which would then be expected to inappropriately arc, causing a short circuit as well as harmful interference compromising control so long as the motor is running.
Large areas could even be blanketed with the material in order to render them drone-free at critical times. One delivery method which is intriguing is a high-Earth orbit atmospheric entry balloon which can drop into targeted areas with little warning for a variety of applications. A release of the web material from a higher altitude would result in a wider area being covered, however it would reduce the likelihood of a successful takedown of a drone. We're probably, therefore, looking at firing the material from other drones; drone versus drone warfare.
