I'd say that it's exactly the fuel, the engine, and the whole chassis of an anti-ballistic missile what's so expensive. An ABM has very little time to do its job. It has to launch instantly, and accelerates at 100g or so, and then must also maneuver at this circumstances. It should also pack enough punch to destroy a warhead which is a relatively small metal needle, built to withstand the mechanical and thermal loads of an orbital reentry.

As a contrast, an ICBM can start with much more soft acceleration, can spend 10-20 seconds preparing for launch without loss of efficiency, and does not have to maneuver much during ascent and the orbital part.