THAAD interceptors look to be ~$100M each from a google search. I remember seeing a leaked document that suggested up to 3 interceptors would be fired at each target to guarantee a kill, but I can't find the document now, sorry.
A Trident-D5 missile is $30M, and can carry up to 14 (small) warheads (which would likely be a mix of larger warheads and penetration aids). Trident is still in production, but note that it's an economical US missile.
So given this very approximate math, ~$2M on offense could soak up ~$300M on defense. The cost comparison would look worse for the defense if you don't assume the (high) cost of a US offensive missile for parity.