The Virginia-class (SSN-774) submarines have unit costs between $3 billion and $4.5 billion, but the U.S. Navy wants the future SSN(X) boats to be cheaper despite their envisioned higher payloads carrying unmanned vehicles and weapons. "We're still in the early time right now. We've set the baseline requirements where we know the big rocks, and we know it's gotta be affordable," Rear Adm. Andrew Miller, the Navy's director of undersea warfare, told Defense Daily after his address to the Naval…