I just do this as a hobby, but for the most part I don't think in terms of cost per quantity of vinyl used. I think in terms of time. If something takes me an hour to do vs something that takes 15 min. I'm going to charge more for the hour one even though it maybe smaller than the one that takes 15min. I will admit though, customers don't think this way. They think the bigger something is, the more it will cost, when in reality, bigger items are often times cheaper to produce in terms of time, and time is the real cost of vinyl decals.