Given these complexities, if NASA decides to pursue this project, PROMISE would likely cost the agency between $700 million and $1.3 billion and not launch until sometime in the early 2030s. This is, emphatically, not a replacement for a detailed engineering cost assessment. If nothing else, PROMISE represents a significant opportunity cost that should to be weighed against the declines in overall science funding and the drawdown of investment at Mars and beyond. Regardless of whether one agrees with every estimate below, one thing is clear: PROMISE is far from a “freebie.” It will consume significant resources that might otherwise be available to pursue established scientific priorities in NASA's scientific portfolio.