Complex sentence: According to the cost-model, source code maintainability is an exponential function of the effort put in the development, i.e. the development effort degrades source code maintainability in an exponential rate.
Split into simpler sentences: According to the cost-model, source code maintainability is an exponential function of the effort put in the development. This causes the development effort to degrade source code maintainability in an exponential rate.