A more thorough examination of the underlying fundamentals, however, discloses many weaknesses in the simple assumptions that made the choice of an experienced builder seem automatic. A maximum requirement limited to only one or two units offered little incentive to a manufacturer already successfully producing in his field, and the disadvantage of the limited quantity was only accentuated by the basic requirement for a technical performance in excess of any standard of the time. Certainly there was no promise of any future quantity business or any other substantial reward. Orville Wright many times stated that they had no desire to produce their own engine, but it is doubtful that they had any real faith in the buying procedure, for they made no attempt to follow up their first inquiries or to expand the original list. Whatever the reasoning, their judgment of the situation is obvious; they spent no time awaiting results from the letter but almost immediately started on the task of designing and building the engine themselves. Perhaps the generalities were not as governing as the two specific factors whose immediate importance were determining: cost and time. The Wrights no doubt realized that a specially designed, relatively high performance engine in very limited hand-built quantities would not only be an expensive purchased article but would also take considerable time to build, even under the most favorable circumstances. So the lack of response to their first approach did not have too much to do with their ultimate decision to undertake this task themselves. The question of the cost of the Wrights' powerplants is most intriguing, as is that of their entire accomplishment. No detailed figures of actual engine costs are in the record, and it is somewhat difficult to imagine just how they managed to conduct an operation requiring so much effort and such material resources, given the income available from their fairly small bicycle business. The only evidence bearing on this is a statement that the maximum income from this business averaged $3,000 a year,[4] which of course had to cover not only the airplane and engine but all personal and other expenses. Yet they always had spare engines and spare parts available; they seemingly had no trouble acquiring needed materials and supplies, both simple and complex; and they apparently never were hindered at any time by lack of cash or credit. The only mention of any concern about money is a statement by Wilbur Wright in a letter of 20 May 1908 when, about to sail for France for the first public demonstrations, he wrote: "This plan would put it to the touch quickly and also help ward off an approaching financial stringency which has worried me very much for several months." It is a remarkable record in the economical use of money, considering all they had done up to that time. The myth that they had been aided by the earnings of their sister Katherine as a school teacher was demolished long ago.