When you're making a manned fighter, not only is there a very large cost penalty for making it survivable for the pilot, but because of that you need to make the entire thing to aerospace specs, which also adds a couple of decimal places to what otherwise would be a complex but not extortionately so machine. Take out the pilot, dump all the weight of the hardware needed for them to control the thing and live to tell the tale, that allows you do downsize it significantly for the same payload. This makes it much cheaper. Since you no longer need to keep a human alive for long enough to do their job and return (or not depending on how good they are vs how good the enemy is) you can make the drone fighter much cheaper still if you drop the human rating of the aircraft to something considerably less exacting. You also have far less in the way of support systems required to maintain a fleet of the things in the field, since if nothing else you don't need to have lots of pilots and support staff on standby. This reduces system cost even further.