Lee Howard has been doing a lot of great research into the subject. I'd recommend checking out his stuff.
A Game Theory of IPv4-IPv6 Transition - »
nanog.org/meetings/abstr ··· ?id=2130The Costs of IPv4-IPv6 Dual Stack - »
www.nanog.org/meetings/a ··· ?id=2075TCO of CGN - »
www.nanog.org/meetings/a ··· ?id=2025Cameron Byrne has made public statements on T-Mobile's v6 Google Group where he has stated v6 only users cost T-Mobile less. I believe they have a problematic issue with needing duplicate v4 address space for failover, which they cannot justify via current ARIN policy, so T-Mo might be an edge case.