If we assume cost is correlated to the time to deliver, then we are really talking about how long the project would take (work with me here ). Of course with Agile methodology this really depends on your incremental delivery approach and what you call “done”. At some level you need a gut level feel of length of time, be it days, months or years, but you would also need to qualify what you mean by “done to completeness”. I would propose that this be amended to read;

#How long will it take to start receiving ROI on the solution?