Do you know how many F16s the US has? They essentially modify aircrafts near the end of their life cycle, adding automate kits. Such a jet is less expensive than new drone and can do more -- fly higher, faster, more complex profiles. They also rarely fly compared to other aircrafts in active duties, manned or unmanned.

This article describes similar modification:

https://www.tyndall.af.mil/News/Features/Display/Article/276...