If those shaheds cost $20,000 which is low balling the cost than that Gepard just eliminated $340,000 worth of drones.
The cost quickly balloons from there for russia as the cost per flying bomb/shahed goes up from having to make them more sophisticated and capable of flying higher to penetrate Ukrainian defenses.
The idea that mass flying bomb attacks are more cost efficient than other types of attacks or that there is no way a defender can’t affordably chew through mass flying bomb attacks with expensive equipment in an affordable way is not really true, under certain conditions it is but under others this is the least efficient strategy possible.
Of course, ultimately cost limits everything, but it’s not always the biggest factor in the short-ish term. It is easy/cheap to increase drone production in a short time, evident from Russia building a big production site after the war started. It is hard/expensive to increase production for advanced missiles, evident from the relatively low numbers of use and constant-ish rate. They (think they) need long-range strike capabilities and they need it yesterday, so drone usage is the sensible option even if it is more expensive.