IPv4 addresses have been more expensive than IPv6 subnets on every dedicated uplink I've gotten prices for over the last two years. In fact, an IPv6 /56 is usually free, or cheap enough to be effectively free since the fee is mainly a NRC for the time to set up the route, if you ask for it and the provider supports IPv6. In contrast, IPv4 addresses often incur a MRC based on the number of usable addresses you request.