One thing that's evident about President Donald Trump's proposal for the Golden Dome missile defense shield is that designing, deploying, and sustaining it will cost a lot of money, at least several hundred billion dollars, over the course of several decades. Beyond that, it's really anyone's guess. That doesn't sit well with some lawmakers, but the Republican-controlled Congress committed $25 billion in July as a down payment for new missile-defense technologies. The White House stated in May that Golden Dome will cost $175 billion over three years, but a new study from a center-right think tank concludes that it is simply not enough to develop the kind of multi-layer shield Trump described in a January executive order. It's also clear that it will take longer than three years to implement the full spectrum of defense capability envisioned for Golden Dome. "The capabilities this level of funding can buy fall far short of what the president promised, creating a multi-trillion-dollar gap between rhetoric and reality," wrote Todd Harrison, a senior fellow at the American Enterprise Institute. In announcing Golden Dome, the White House said the program must defend the US homeland against many types of aerial threats, including ballistic, hypersonic, and advanced cruise missiles, plus "other next-generation aerial attacks," a category that appears to include drones and shorter-range unguided missiles. A program with few parallels Harrison's paper, titled "Build Your Own Golden Dome," provides a framework for understanding the costs, choices, and tradeoffs facing the Pentagon as officials decide what exactly a homeland missile-defense shield will entail. And there are a lot of decisions to make as defense officials begin to define Golden Dome. "Its cost hinges on the level of geographic coverage, the types and numbers of threats it must address, and the degree of resilience it is expected to achieve," Harrison writes. "As this analysis shows, even slight changes in these parameters can alter costs by hundreds of billions of dollars.