I agree.
Wouldn’t bursts of 50 cal be sufficient?
Sort of a “small caliber” Phalanx system.
On a unit cost basis they would be a lot cheaper. They would not need to be fancy - maybe a quad .50 cal mounted on a truck. But there are other considerations. You would need a lot of them to defend an area as the range of effectiveness would be a few kilometers only. So if you want to defend a 10 kilometer wide city you might need 6 dozen or so to just to give two layers of all around defense. Terrain around the objective to be defended is a big consideration as well. If the terrain is fairly flat then you might not need a lot of them but if there are intermittent hills and valleys or a lot of woods you may need a lot more of them to get adequate coverage. Acquisition may be a real problem with relatively small drones, although a centralized detection capability might be able to detect and assign targets to individual low tech flak units like described above.
If we spend $150K per flak unit to arm the trucks with quad 50s and give a simple capability to accept targeting data from an acquisition facility, the cost of defending our notional city is about $10.8M plus the acquisition capability (maybe $25M). Ammunition for the system is dirt cheap.
Now suppose we have a missile based short range (about 15 km) air defense system to defend our city If we place three missile launcher units ($20M each, includes acquisition capability) with 8 missiles each (at $200K per missile) then the setup cost is $64.8M. Ammunition will be somewhat costly if a lot of drones are launched.
Many other factors come into play here but the problem you pose is fairly complex and is filled with educated guesses as to the ‘best’ way to defend an objective against drones.