Something I have witnessed in the past 40 years is the cost of building. Materials have escalated in cost, as almost everything but the cost of shipping has sky rocketed. I the mid 1980’s ordering a 2′ x 4″ piece of alum sheet, say .030 thick, material would cost about 10 to 12 bucks and shipping about 10. As the years went buy and in 2011 costs were about $ 15 for aluminum and $ 15 for shipping. Last week needed a sheet, sheet was about $ 16 but shipping was about $ 45. It is not the cost of materials that is driving the costs up but shipping. I live about 300 miles from aircraft spruce and it is getting to where it is more cost effective by saving up my orders and driving down to pick them up. It is critical to plan ahead and allow for extras, getting an extra bolts, nuts, washers, other materials, that may be required now or later is certainly better than need and another 20 washers, they cost .80 cents but shipping is $ 4.00.
Something else is critical is to place those newly purchased items so they do not get lost or disappear before you need them. Spending a few bucks for storage system is a good investment.
If a person built a flying machine these days one piece at a time and ordered one piece at a time when complete he could say, here is my $ 30,000 Legal Eagle, $ 10,000 for materials and $ 20,000 for shipping those parts to me. Used to be $ 10,000 materials and $ 1,000 for shipping. Where will it be in another 10 years?