One of the major concerns in the DABS development program has been the cost of the DABS transponder. In order to realistically assess the impact on transponder cost of the many alternative techniques and design choices being considered for DABS, four study contracts were awarded to avionics manufacturers to design and estimate costs of special circuitry (in Phase I) and complete transponders (in Phase II). This report summarizes the major results of these design/cost studies, which cover general aviation, military, and air carrier transponder designs (including a retrofit kit for the military APX-72 transponder). The transponder design/cost studies have had a marked influence on the design of the DABS signal and message formats. Since the cost studies were basically intended for comparing link options for DADS, the transponder specifications used in these cost studies do not correspond in detail to current DABS transponder specifications. Therefore the cost data contained in this report cannot be taken to be completely representative of the cost of the finally specified DABS transponders.