As far as I can tell it's the other ways around: IPv4 is getting more costly
Example: AWS started to charge for IPv4 addresses a few months ago - a IPv4 address now costs around $3.6 per month
Context:
People have been asking for IPv6 Support on GitHub since years (probably a decade by now)
... and someone even got so annoyed that they decided to setup a dedicated website for checking this: https://isgithubipv6.live/
Context: After evaluating the DockerHub Verified Publisher Program and receiving a mail with "it costs as little as $5k per year" I have come to the conclusion that it's not worth it
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As far as I can tell it's the other ways around: IPv4 is getting more costly
Example: AWS started to charge for IPv4 addresses a few months ago - a IPv4 address now costs around $3.6 per month