"Your costs may vary" should be a mantra repeated whenever you're contemplating a strawbale structure. Strawbale houses are not a panacea for affordable housing, though they can offer low tech which can (but doesn't necessarily) decrease labor costs. Let's look at some expenses from a real house. The following costs were sent to me by Bob Munk, whose house is featured in The Straw Bale House book. The costs are in dollars spent per square foot, and I've calculated the resulting percentages for each category.
Some additional information about Bob's house will help your interpretation of these numbers.
Bob noted that some people leave out architect and all site development expenses from building costs per square foot, which would exclude costs for the well, septic, and site preparation. Bear in mind that the percentages below are for the total of all the costs listed.
Again, no two people would find their percentage totals to be the same. But the list is helpful for focusing both on "tiny" costs, and on the bigger ticket items that might be reduced.
The top four costs in this case--each over 10% of the total--were concrete, framing, plumbing, and doors/windows. These four categories total 45% of the total cost, an amount which invites investigation by a potential builder.
The four major categories were followed by plaster; floors; electrical; kitchen; and roofing, with percentages ranging from 8.5 to 5% of the total, and totalling, as a group, about 33% of the total building cost. Thus the top nine cost categories amount to 78% of the total.
Not all of the categories listed above will be used or needed by all builders. But rather than lingering too much on these particular figures, the list might be better used to identify categories of expenditures that might contain the greatest amount of "fat", or in focusing design tricks that minimize certain costs. For instance, one way to reduce plumbing costs (in addition to paring needs to a minimum) is to centralize piping runs, that is, to locate all plumbing fixtures in one area rather than spreading them throughout the house. Another trick is to make it relatively for the plumber to install the piping by designing in (at least temporary) access to the plumbing core area. Contractor time, after all, is big money.
All cost categories respond to such attention. Conversely, categories which are ignored tend to overflow their intended cost limits. The total cost per square foot for this house, by the way, was $108.88, which is a moderate amount, with the lower limit typically being about $50-60/sq ft, and the upper limit being $200 and higher. One cost that is not present in this list (and which might be very significant) is the cost of land (and possibly road access costs).