Demolition, relatively speaking, isn't that expensive. Construction, design, painting, electrical, plumbing, wiring ... none of that is part of demo. Wreck, smash, bulldoze, done. By comparison, it just a lot cheaper than building. Not "cheap" per se, but not expensive when taken in the context of $2+ billion.
Just as an off the wall example, When Yankee Stadium was demolished in 2009/2010, the cost was reportedly $22 million. That's a HUGE undertaking in a highly populated urban environment where everything costs more to do, and they weren't permitted to do any implosions, they actually had to tear it down piece-by-piece. If you look at other large-scale demolition projects like that (stadiums and such), they rarely go much above $10-$20M or so. And the demo work that would need to be done at DHS shouldn't go much beyond that scope/scale. Even if the demo work went several times that (unlikely), it's still just a small dent in the budget.