This is accurate. The opportunity cost has already been sunk by players who know how to achieve this market return rate. The assumption is that as players level up, they gain more understanding of the in game currency and how to earn it in the most optimal way. Thus once the opportunity cost is paid, (via time spent finding the most efficient way to make inf) it becomes trivial. But from a new player perspective who hasn't spent anything on that aforementioned opportunity cost, then absolutely, what you are saying is accurate.