Even so, SpaceX can launch 50 satellites in one launch, and each suborbital interceptor can only hit one. And the "if it had navigation" part is doing a lot of work in that sentence; a satellite interceptor kill vehicle is not easy to build. Are 50 of them cheaper than one Falcon 9 launch with a 15x reused booster? I think "maybe" is the only possible answer, but probably the cost of killing all the satellites that way would be around the same order of magnitude as putting them up in the first place. You would need to build multiple whole factories just to make the thousands of interceptors you would need.