Q: How do you calculate the cost of migrating from IPv4 to IPv6 for a large network size? And what is the expected ROI?
A: This greatly depends on your network layout, your systems, your applications and services, and your strategy. For instance you need to determine whether you want to go for a dual-stack network or whether you plan to go to a pure IPv6 network as soon as possible. In order to get to numbers, you need to work on a migration concept which will analyze your current infrastructure, create a design for the future and then evaluate each single aspect to find out what it takes to get there. Once those factors are determined, the time frame you have to reach your goal plays an important role, also. The sooner you start, the more digestible you can plan the single steps and the cost associated with it.
The US Department of Commerce (DoC) did an analysis in 2006 and came to the conclusion that every dollar ($1) invested in IPv6 creates a 10 dollar ($10) return. If you're interested in this study, here it is: Technical and Economic Assessment of IPv6.