In fact some of the earliest adopters of IPv6 were Google, Microsoft, Netflix. Companies who when you're considering the problem of (N * a few bytes) have a very large N so are the most likely to have material costs from it. Yet even to them, it's a rounding error.
For Netflix the cost is actually especially low. Cost as a percent of bandwidth is (a few bytes / packet size), and when you're streaming enormous media files packets are almost always max size.
