TIME AND MONEY
All of this, of course, is not the end of your expenditures on the addition. It is inevitable that there will be items you forgot or bought in insufficient quantity, 2X4s for example. No matter how many you thought you needed, you are certain, dead certain, absolutely dead certain to need more. 2X4s to an addition are like cells to life. It is the basic building block. You will find yourself using them for everything, from bracing a free-standing rafter to venting your frustration on a wall frame that you built one inch too short. They will brace plumbing, make fire stops, frame soffits, etc.
There are probably many items that you intentionally did not purchase on your initial buying spree: electrical supplies, plumbing items, and so on.
Sometimes all you need are a few pounds of nails. Whatever it is that I need, my wife wants me to treat it as a major purchase. She would like me to take five bids for each item. Imagine that I need a drywall hammer and a dozen 2X4s. The difference in cost from one lumber yard to another is not likely to be great on one of these items. Say it is twenty cents per 2X4. That would mean an additional $2.40. It would take me an hour to do all the calling around necessary to find the lowest price. How much is my time worth? I would estimate it at slightly more than $2.40 per hour. My wife thinks that it is in the neighborhood of 38 cents.
Thus, I do a lot of driving and calling around, pricing materials. If, as a result, I save more than 38 cents, my wife considers it time well spent.