An arbitrary number is all you could ever get no matter how good the reason.
Eventually you'd have to consider the actual cost of installing sprinklers and the actual cost of buildings burned down and compare that to the "cost" of lives lost or saved. And the dollar value placed on a life will always be nothing more than arbitrary.
There is no accurate way to solve this problem so 300 is as good a guess as any to me. Besides, a building with an occupancy of 300 persons is a much smaller building than you may realize.
