Size and range.

Strike drones have to be able to carry a fairly large warhead (or are only good at hitting people and not things) and they have to fly quite a long way to get at things like reserve assets and logistics. So they are quite big, with quite a lot of fuel etc. Big things tend to cost more. In this case I can imagine that an interceptor that has a range of 10k and is 5% of the size of the strike drone would be able to knock it down and would be able to do so well away from its target.

Dunno how anyone can "know" unless they "know" and then they are not talking. But, it seems plausible that something with 10% of the range and 5% of the mass would cost 10% or less.