Ha! Even when the initial cost of a new airplane is $60Mil or $250Mil, whether one is buying one (perhaps a BBJ) or thirty, to upgrade a sand fleet, there are so many questions. While virtually no hardware is changed, the operating software is different for every buyer. A few standard packages are already certified, but unusual requests often require engineering and in-flight proof - and yes, that is expensive. Boeing will do it is one pays enough (or buys enough airplanes) and Airbus is reluctant, but can be pushed. Certificated modifications to the 'basic' operating software for today's new jets is certainly possible, but it can be extremely expensive. Smart BBJ operators accept the best off-shelf package; huge operators (sand airlines?) may buy a few changes to accommodate temperatures, and smart airlines buy what is available. Ever wonder why "Launch Customer" is important to some? For the most part, they get software modifications free, in exchange for their fleet-wide testing. If they already want a dozen or more copies, and generally fly routes within the marketing envelope, this can be a sharp move. The one or two-off operator cannot afford it.
Buying new airplanes at multip-$millions per copy is not easy. Unless buying to another's spec., it take at least 9-12 months to wiggle out the finer details - and ever change has a price. (Glad that I no longer sell them! I may keep my hand in, but I hate airplanes - or love them... I've never been sure which...)
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