As I recall, the ROI was going to be achieved through fuel savings from the reduced weight and maintenance costs. The maintenance savings was big. They went from something like 27 line-replaceable-units to six. That also cut hundreds of pounds from the a/c. The mod back then I remember seemed very cheap at around $250k per aircraft. Do that to over 200+ aircraft and the expense up front is large. But the savings from the weight and mx, would pay for itself in 3 years. However, back then the NWA merger hadn't gone through and they wanted ROI's that were shorter. Perhaps now, they will look at the longer term savings now that these aircraft are here to stay till at least 2020.