IPv6 addresses are practically endless, therefore their value is practically 0. ISPs justify charging extra for static IPv4 because IPv4 addresses do have a value.
If ISPs charge for static IPv6, then one of them could just give that service for free (while keeping the rest of the prices the same as their competitors). That would get them more customers while costing them nothing.
EDIT: I can’t give you an example of an ISP that offers free static IPv6 because there are no ISPs in my country that offer IPv6.