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Development costs of a few recent spaceflight instrument electrical and electronics subsystems have diverged fvom respective heritage cost model predictions. The cost models used are Grass Roots, Price-H and Parametric Model. These cost models originated in the military and industry around 1970 and were successfully adopted and patched by NASA on a mission-by-mission basis for years. However, the complexity of new instruments recently changed rapidly by orders of magnitude. This is most obvious in the complexity of representative spaceflight instrument electronics' data system. It is now required to perform intermediate processing of digitized data apart from conventional processing of science phenomenon signals from multiple detectors. This involves on-board instrument formatting of computational operands from row data for example, images), multi-million operations per second on large volumes of data in reconfigurable hardware (in addition to processing on a general purpose imbedded or standalone instrument flight computer), as well as making decisions for on-board system adaptation and resource reconfiguration. The instrument data system is now tasked to perform more functions, such as forming packets and instrument-level data compression of more than one data stream, which are traditionally performed by the spacecraft command and data handling system. It is furthermore required that the electronics box for new complex instruments is developed for one-digit watt power consumption, small size and that it is light-weight, and delivers super-computing capabilities. The conflict between the actual development cost of newer complex instruments and its electronics components' heritage cost model predictions seems to be irreconcilable. This conflict and an approach to its resolution are addressed in this paper by determining the complexity parameters,complexity index, and their use in enhanced cost model. IntroductionThe representative complex spaceflight instruments and prototypes are, for example, the Ocean Carbon Ecosystem and Near Shore Processes Mission (OCEaNS) optical instrument with high volume and high rate data streams from its focal planes and requiring on-board digital time delay integration. The non-optical Magnetospheric MultiScale Fast Plasma Investigation (MMSIFPI) instruments require on-board intensive computation of the Burst Quality Index. The Laser Interferometer Space Antenna (LISA) may require quad precision floating-point arithmetic computations and the Solar Viewing Interferometer Instrument Prototype (SVIP) is based on image processing within fast attitude control loops. A complex instrument is one that has large arrays of large detectors at fast readout rates, producing signal volumes and rates of order of magnitudes higher than heritage instruments and instrument based random access memory around 500 megabytes, and requiring non-trivial analog signal and digital data processing on-board the instrument.There is a solution -to fly a super-computer, but obviously, it is not feasible. ...