A new Phantom weighs about 3 tons. To armor that (2-3" thick ballistic steel or something else for every body panel, bullet resistant glass is also heavy as hell) brings a car probaby 10 tons easily, if not significantly more. So even besides the cost, you would almost certainly have to re-gear the transmission and retune the engine (if not put in an entirely new, stronger engine, depending on the car/native engine) to deal with all that new weight.
"I'd hate to die twice. It's so boring" - Richard Feynman, last recorded words.