If the realistic estimates of the population of an ecumenopolis like Coruscant, almost completely covered in skyscrapers miles high, are to believed (as opposed to the officially published figure which seems way too low), then perhaps a quadrillion people died in the destruction of Hosnian Prime, a similarly urbanized planet: 100000 to 200000 times the current earth population.
Alderaan seems similar to earth now, so much much fewer were killed by the Death Star.
Tell a realistic story of galactic war in which death tolls in the quadrillions, even a daily death toll in the quadrillions, is par for the course. Before the planet busting weapons, maybe Base Delta Zero against an ecumenopolis.
Incidentally, Starkiller Base illustrates the weakness of Saddam Hussein's unmoveable supergun: fire it just once and its location will be revealed to the enemy, who will come and destroy it. The Death Star, being mobile, was a much better design.