Before they were falling out of fashion, there were relatively "cheap" versions produced in larger numbers (like 400+ Gepards for Germany, for example, no idea

Before they were falling out of fashion, there were relatively "cheap" versions produced in larger numbers (like 400+ Gepards for Germany, for example, no idea how many of the 35mm guns were made), if I recall correctly, also, you don't need thousands of AA guns to protect an object. The current skyranger 30 system is $5+M I think, but I don't think you need necessarily luxury systems to defend against rather simple drones (and it has low production numbers, I believe).