In what world would a specialized, niche, high precision, high readiness rocket meant to loft a very advanced interceptor munition into an extremely high velocity interception ever be cheap?

These things are closing at like mach 20. Physics says that's hard to do. That means it's expensive.

For reference, $75 million is in the realm of a Falcon 9 launch, which is a very cost optimized platform that doesn't have to place a very very precision instrument payload in a very very specific point in space to prepare it for a high energy, extremely difficult interception.