So IPv6 is expensive, but NAT is not? What makes IPv6 so expensive?
That's not my experience, anyway. Of course, there aren't many providers offering IPv6 yet, but the one I know of in my area does not charge anything extra for IPv6. It's part of the package, take it or leave it. Tunnels are free, too.
How do ISPs respond when customers complain they can't reach IPv6-only sites? Sure, this won't happen for a while, but eventually it will.