The $5 million per WAR comes from what teams spend on free agents as a whole compared to those players' WAR values as a whole. Anywho, a replacement level team is considered to be 45-50 wins, that would be the starting floor in your example, not zero. Teams obviously obtain considerable value from their pre-arb/pre-FA players making a fraction of their open market value, so the overall value per win at the team level isn't $5 million because not all players come out of free agency. Theoretically, if a team did build itself entirely out of free agents, with no indentured servants, they would end up paying in the neighborhood of that $5 million per win to go above those 45-50.