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.

> If I recall correctly, I have to pay _extra_ for an IPv6 address.

That's weird, here we get a /48 v6 block by default and one /32 v4 address. You'll never need more v6 addresses, but each v4 address comes at a monthly fee.