Processing of MOS ICs is less expensive than bipolar ICs primarily because they:

Concept:What is an Integrated circuit (IC)It is an assembly of the electronic components, Fabricated as a single unit, in which miniatured ACTIVE devices [e.g. transistors and diodes] and passive devices [e.g capacitor and resistors] and their interconnections are built UPON a thin substrate of semiconductor material (typically silicon).The resulting circuit is thus a small monolithic chip, which may be as small as a few square centimeters or only a few square millimeters.The individual circuit components are generally microscopic in size hence It is also called a microelectronic circuit or microchip or chip.Advantage of MOS IC, over BJT IC1) It requires less space i.e. higher PACKING density2) Manufacturing process is simple3) Operating power is small4) Smaller leakage current5) Higher frequency STABILITY. NOTE: The SPEED of operation in BJT ICs is greater compare to MOS ICs.