The cost is driven by a few factors:
- magnetically detecting a very fast moving non-magnetic object (lead bullet), maybe they can do this with inductive sensors? Still not trivial to get the exact moment the bullet is at an exact location, light is much easier for this.
- survive the muzzle blast of firearms. That's a pretty tough one, too.
Probably on top of that the whole contraption is pretty rugged as well. At least it looks the part.