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.
Wiki says that $800M gives you a battery of 6 launchers (each equipped with 8 missiles), 2 tactical centers and 1 radar. So the upper bound for one missile is 16M. Still very expensive of course, but not 100M expensive.
Pretty sure that price for D-5 doesn't include the cost of the W-88 warhead. I would assume at a minimum those would be $10M a piece. Still doesn't make it more expensive than THAAD however.
This is hilariously cherry-picked. THAAD is extremely expensive if that's the case. Wikipedia says you can get a Patriot for just $4M. Also SLBM's as actually deployed aren't using maximum MIRV counts, both for treaty and role reasons. Wikipedia says 4 RV's on average for Trident, FWIW. But the big whopper is that you're forgetting to account for the $4B SUBMARINE (just $200M per missile though) required to deploy those missiles in their cost!
Good job though. Honestly the numbers are closer than I'd have expected. But ICBMs remain more expensive.
I chose Trident because I wanted a fair(er) comparison of costs due to purchasing power. If you want to use North Korean ICBM pricing, it's $30M[1]. THAAD is very expensive, that you're right about...but it's also the only interceptor that has a chance of success against an ICBM. Please note as well, trident is typically deployed with far fewer warheads likely among other reasons because it doesn't need to be an interceptor sponge, per its role. If you're North Korea with a small arsenal, every one needs to count against an opponent with THAAD.
