# input size = 1 , estimate is 0.00009999999999976694
var explain = db.foo.find({a:1}).sort({a: 1}).limit(1).hint({$natural: 1}).explain('executionStats');
explain.executionStats.executionStages.costEstimate - explain.executionStats.executionStages.inputStage.costEstimate;

# input size = 10K, estimate is still 0.00009999999999976694
var explain = db.foo.find().sort({a: 1}).limit(1).hint({$natural: 1}).explain('executionStats');
explain.executionStats.executionStages.costEstimate - explain.executionStats.executionStages.inputStage.costEstimate;