does anyone ever check "actual costs"
My office does about 10 new constr. per month. Usually these have a bid (many are owner doing part of work). So I have direct bids, very useful for individual items like well, septic, as well as building costs. Also, have sales of specs and good land value support to extract costs from market. After a few from each local builder you get a real sense of differences in how each price their work.
One interesting note. A builder's wife became a real estate salesperson a couple of years ago. After a while she was able to find the "right" out-of-town appraiser, so now her husband's houses sell for about 10% more than they used to compared to other builders. 90% of his buyers are from out of state, members of his church, and/or FHA qualified. She explicitly asks that I and another appraiser (local) NOT be used. Only 1 that has a tendency to be high and her pet from across the county are allowed.
Using the market sales, I have worked backward thru the cost books and Boeckh was the closest to what our market was doing. Natl cost books give you quite a bit of wiggle room, but are not far off. M & S, Like Ed sez, seems to be a little high out here in the sticks for an average house, but I tend to find that for the high end houses, they are pretty much on to a little low.
If the Cost App. is critical, it doesn't hurt to test two sources. I do this quite often for commercial buildings, especially if there is a large difference in what I am figuring and what the owner of a newer Comm. building said it cost to build.