Which one? You are doing a lawn for $40. It takes you 1 hour. And your costs are $15. $25 profit. Someone (salesman) comes to you and says I can sell you a gadget that will cut your time in half. It will cost you $15 to use it on that lawn. So your time will drop to 30 minutes but your cost will be $30. Would you take them up on the deal? What if it cost you only $5 per lawn to cut the time in half? At what point either higher or lower than $15 (additional cost) would you CHANGE your mind?