If you are willing to wait, you pay nothing. However, let's keep in mind that this evaluation is based on the private transfer market place for IPv4s not based on actual RIR costs.

You must be an RIR member to hold IPs and there are membership dues that you must pay each year to maintain your allocation. Once you are a member of an RIR you just have to make a request and at least with ARIN that request and fulfillment is free.

ARIN’s free pool of IPv4 address space was depleted on 24 September 2015. As a result, we no longer can fulfill requests for IPv4 addresses unless you meet certain policy requirements that reserved blocks of IPv4 addresses for special cases. https://www.arin.net/resources/guide/ipv4/ ie. you have virtually no other option than to buy on the private market

ARIN is the first to deplete. I think other RIR still IPv4 left.

But the wait is long and everyone is rationing addresses so any free allocation is unlikely to be very large.