In a new study on Six Sigma the savings to cost ratio ranged from 2.6 : 1 through to 2 : 1. As for cost savings as a percentage of revenues, the average was 1.7%. This study showed clearly the impact of Six Sigma and its ROI.
As a rule of thumb a company with $100 million in sales could expect $1.5 million of direct savings per year of implementation.
(Costs and Savings of Six Sigma Programs: An Empirical Study, Quality Management Journal, October, 2012)