Small drones pulled into jet engines can do serious damage.
A drone with software that can detect the air intake of the F35 jets and position itself to be sucked in to the engine would be relatively inexpensive and be programmed easily in the near future. No radar needed, just a cheap camera and software. No bomb, the drone itself is enough to cripple the engine. No operator intervention required. The F35 pilot won't even see the drone before it cripples his single engine.
The comparable cost - $1,000 to $70,000,000 or 70,000 to 1.