Transfering IPv4 prefixes comes with a 2 year transfer restriction period.

IPv4 addresses currently cost about $50, and you have to buy an entire subnet at once, minimum /24.

It makes perfect sense to start charging for them if you're running out of them and don't want to buy additional prefixes.

Not to mention that there's also additional cost involved in case someone was using that particular address (or even worse multiple addresses from the same /24) for spam or malicious purposes, because that means that the entire prefix is currently trashed and there's some effort involved for it to be removed from all the independently maintained blacklists etc.