Author here. The $75M is specifically for Ground Based Interceptors (GBIs). This is the U.S.'s ICBM mid-course interceptor. There are other interceptor types in the current U.S. arsenal:

Patriot PAC-3 (~$4M): Nations burnt through 600-800 in the first few days of Operation Epic Fury. There are reports that they're being used for drone defense.

SM-3 (~$10-30M): Ship-launched

SM-6 (~$4-5M): Ship-launched

THAAD (~$12-15M): Terminal phase, high altitude

GBI (~$75M): intended for interception of ICBMs (reported as the hardest type of missile to intercept)

Each type of interceptor is optimal for certain type of threats, which is yet another constraint on the optimization problem.