Exactly, this is not a one for one trade off.Let's say they're $10K each. A truck-load is $480K with 48 of them. 5 truck-loads is is $2.4M and they launch 240 drones. 10 truck-loads is $4.8M and 480 drones. If they're enough to overload target defenses and the autocannons can't take enough of them out, the ones that strike can incapacitate (at the most critical moment) or destroy a target that cost hundreds of millions or over a billion dollars.
On top of that the drone has a much longer range and standoff from AA round. The AA round will also need to be guided and manned. If the attack happens at night, the number of rounds expanded in order to destroy a single drone quickly mounts.
Lastly, there attacker will always has a advantage of initiative. Hence, the radar/optical operator needs to be manned 24/7, while the drone operator can choose the time of engagement.
Once all these factors are tallied in, the drone is likely 1/100 the cost of the defensive system in total gear, logistics and personnel.